Why Your RV Air Conditioner Is Not Cooling in Las Vegas Heat

Ranked by how often each one turns out to be the real cause on a coach that arrives with the complaint "the AC is not cooling". The order matters, because it tells you where to spend the first ten minutes.

1. Restricted airflow. A filter loaded with desert dust, a return grille with something in front of it, closed ceiling registers, or an evaporator coil that has not been cleaned in years. The unit is fine. It simply cannot move enough air across the coil to do its job.
2. Asking for more than the unit can give. Rooftop air conditioners are rated by how far they can drop the air temperature, not by the number on the thermostat. On the hottest days a healthy unit reaches its limit well above the temperature you want. Section three explains the arithmetic.
3. Power supply. Low voltage from a long or undersized cord, a tired park pedestal, or a generator carrying more than it should. The compressor starts weakly, stalls, or trips the breaker, and the symptom looks exactly like a failing unit.
4. A failed electrical component. Most often the start capacitor, which gives the compressor its kick at startup. Less often a fan motor, a relay or a control board. These are real faults, but they are parts, not the whole unit.
5. The sealed system. A compressor that will not run, an evaporator that ices over on every cycle, or refrigerant that has leaked out over years. Least common, and the one case where replacement of the unit is usually the realistic answer.

The first three are things you can find and often fix yourself. The last two are where the guide sends you to a technician. Everything below follows that order.

A rooftop RV air conditioner does not produce a temperature. It produces a temperature difference. Air comes in through the return grille, passes over a cold coil, and leaves the supply register somewhere between 16 and 22 degrees cooler than it arrived, depending on the unit, the humidity and how clean the coil is. That figure is the whole capacity of the machine. It does not get bigger because the thermostat is set lower.

Now apply that to a Las Vegas afternoon. With the outside air at 115 degrees and the sun on a roof that is hotter still, the coach is gaining heat through every wall, window and roof panel faster than it does anywhere else you will ever park it. The unit is pulling 20 degrees out of the air it recirculates while the building leaks heat back in. Where that balances is typically somewhere in the mid to high 80s inside. That is not a broken air conditioner. That is a working air conditioner at the edge of what the physics allows.

This is why the most useful tool for the job is a cheap thermometer, not a screwdriver. Hold it in the return grille and note the reading. Hold it in the nearest supply register. If the second number is 16 to 22 degrees lower than the first, the unit is healthy and the rest of this guide is about heat load and power, not repair. If the spread is small, the unit is not delivering and the fault is in the airflow, the electrical side or the sealed system.

What actually moves the cabin temperature on those days is heat load, and most of it is under your control. Pre-cool from early morning before the walls soak up the sun. Close the day-night shades and drop reflective covers over the windshield, which on a Class A is the largest single heat gain on the coach. Deploy the awning on the sunny side. Park with the smallest face of the coach toward the afternoon sun. On a coach with two units, run both from the start rather than waiting for the first to fall behind. None of this is a repair. All of it is worth more than a service call on the hottest week of the year.

A compressor is a motor under heavy load, and motors care about voltage. A rooftop unit that draws in the low to mid teens of amps while running draws several times that for the instant the compressor starts. If the voltage at the coach sags during that instant, the compressor starts weakly, stalls, or trips the breaker. Everything about that looks like a failing unit, and none of it is.

The usual causes are on the supply side. A long extension cord, or one that is too light a gauge for the run, drops voltage under load: a cord that is fine for a converter and lights is not fine for an air conditioner start. A household 15 amp outlet through an adapter sits at the edge of one unit running and well short of one unit starting. A park pedestal with a breaker that has tripped and been reset for years can deliver weak power while showing no fault. On a 30 amp coach, a water heater on electric, a converter charging a low battery bank and an air conditioner compressor kicking in at the same moment add up to more than the service can carry, and the air conditioner is the one that loses.

Generators have their own version. A portable unit that runs a single rooftop air conditioner comfortably may not start it in high heat at altitude, and running a second unit or a microwave alongside it is the reason the breaker trips. Soft start kits, which reduce the surge at compressor start, exist precisely for this and are the honest fix when the generator is the limit.

The test is simple. A plug-in voltage meter at the coach inlet, read while the compressor starts, tells you whether the supply is the problem. Voltage that dips well into the 100s at that moment is the answer, and no amount of work on the roof will change it.

If the filter is clean, the airflow is clear, the supply voltage holds, and the unit still does not deliver its 16 to 22 degrees, the fault is inside the machine. Four patterns cover almost all of it.

The capacitor. The fan runs, the thermostat calls, and the compressor hums for a second and then clicks off or trips the breaker. The start capacitor stores the charge that gets the compressor moving against head pressure, and capacitors are consumable parts that age faster in heat. This is the most common mechanical fault on a rooftop unit and one of the least expensive to put right, which is exactly why it is worth having checked before anything larger is condemned.

The compressor does not engage at all. Fan runs, no hum, no compressor. That can still be a capacitor, or a relay or control board that is not passing the call, or a compressor that has failed internally. The first two are parts. The third is usually the end of the unit.

The evaporator freezes. Ice on the coil, weak airflow that gets weaker, then no cooling at all until it thaws. Low airflow causes it, running on cool nights causes it, and a refrigerant charge that has dropped over the years causes it. The first two are fixable at the filter and the thermostat. The third is not.

Refrigerant loss. Rooftop units are sealed systems. Most have no service ports, are not designed to be opened and recharged, and lose their charge slowly through a joint that has flexed on the roof for years. The unit cools a little less each season until one summer it cannot keep up at all. There is no top-up. The realistic options are a replacement unit or accepting reduced performance.

This is the point where a checklist stops paying off. Telling a weak capacitor from a failing compressor, or a low charge from a dirty coil, takes a meter on the capacitor, a clamp on the compressor current, and a gauge set where the unit allows one. Our RV air conditioner repair page describes what that diagnosis involves and what can and cannot be repaired on a rooftop unit, so you know what to expect before the coach comes in.

Almost every cause above is either airflow or heat, and both respond to maintenance that costs nothing but an hour on the roof before the season starts.

If you would rather have all of that done with the rest of the coach in one visit, the air conditioning is one section of the 45 point RV inspection, which checks the filter, the shroud, the gasket and the drain alongside everything else that fails in the heat.