This system on the 1990 Sunflyer is independent. There is no switch that has to be on for the system to operate. I have no idea about any other model, as this is the first MH we have ever had. I'll run down the steps I went through for anyone else that might have a similar issue.
1. Power is on at the dash.
The "Compressor Operating" light should be on. (That doesn't mean the compressor is really working...it means there is power at the solenoid and that the solenoid has switched to the "on" position). If this light is inop I would check power at the solenoid. The solenoid is on the back side of the motor baseplate and looks exactly like a starter solenoid from my old Mustang. I had to tap this to get it working right the first couple of tries.
If there is no power at the solenoid you'll have to trace your power issue back to the dash. Might be wiring gone bad between the dash and the solenoid.
2. The compressor piston wheels freely rotate. Power must be off at the key for this step!
There was a blue triangular cover on the face of the compressor held on by a single 3/8" nut. Remove the nut and the faceplate will come off. Go easy with it. It's plastic. Once the cover is off you will see 2 Piston wheels at the bottom and the drive sprocket for the compressor motor at the top of the trangle. On my model this is a belt driven system. Make sure the belt is not dry rotted or worn out. Replace as needed. (I think in a pinch a vaccum cleaner belt might work as a suitable sub if your on the road). If it won't rotate with the belt on, slide it off and check the wheels individually. That should tell you if the Pistons or the moter are the issue.
3. Use an Aux compressor to put pressure in the holding tank of the compressor.
On my model there is an auxilliary chuck on the back side of the primary that you can hook another compressor to or go to a gas station and use theirs. I noted that there was no movement to the pillows until the pressure was above 50 psi in the holding tank. You should be able to hear any leakage in the compartment from the tank. Luckily in my case this was not an issue.
4. Check to see if there is pressure at the air pillows.
On the 1990 Sunflyer they are located directly inline with the duallys on the aft side of the leafsprings. When you are there look directly below them. You should see a metal "box" with a bladed relief valve in the bottom. With presure in the holding tank you should get a shot of air from both sides if you are getting pressure to the pillows. If not there is a problem likely at the holding tank with the shuttle valve that allows pressure into the system from the tank. If you hear a leak it can be located by using a soapy water solution in a spray bottle. Be sure to rinse the area after marking any leaks as soap and rubber don't like each other very much over time.
This is very basic stuff, but I was able to isolate my compressor issue down to the compressor motor itself. There had been some chaffing or something had been shoved into the wires that caused the casing to be worn and the positive and negative wires going into the motor itself to come into contact and fuse. I'm taking the motor off, opening her up and rewiring this. Hopefully the motor isn't completely shot and can be salvaged. If not It won't be overly expensive to replace it. Looks a lot like a riding lawnmower starter motor...hmmmmm different rod, threaded bolt in the drive end.......
I am forever figuring a way to make it work with what's at hand for the short term. I once made it 25 miles on the highway to a parts store with a broken alternator belt in my '79 Ford F-150. Used a pair of pantyhose to get me that far.......