[Parisjet]

Sure !

Voltmeter should be set to AC.  Keep in mind you will be exposed to 220 volts with the ATS open.

The very first thing to test is the plug that you are plugged into. Put “testing 50 amp RV plug” into your search engine and watch any of the 100’s of YouTube videos on how to do it.

I suggest you unplug the coach before you remove the lid. That way if you get fat fingers you won’t drop any screws into hot electrical circuits. Take some time to familiarize yourself with what you are looking at before you plug it in.

You are going to see two square contactors with three terminals on each side. The middle terminal will be a white wire, the neutral. The other two will be black and red. Those are the two hot legs.

Identify which of the wires is coming from the shore power by plugging in the coach. The contactor associated with the shore power will close. You should see a small square in the center of the contactor. It will be depressed when the contactor is closed. 

The wires closest to you will be the input wires. One from shore and one from generator.

On the three input terminals to the contactor, measure from neutral(white) to one of the hot legs (black). You should get 110 or so, measure from neutral to the other hot leg (red). You should also get about 110 or so. Now measure from one hot leg to the other(black to red). You should get 220 or so. This step confirms the power is getting from the pedestal to the ATS.

Now go to the other side of the contactor that is engaged or activated. It will be the other side of what you just measured. Repeat the measurements above. If the contactor is properly transferring power, you should get very close to the same readings.

Come back with those numbers, and we can tell if more testing is required. I say that because of your statement that the systems work when on the generator.

If the voltages are not correct on the input side, then the problem lies between the ATS and the plug. It could be loose connections on the ATS, in the plug, or bad contacts in the cord reel. It could be your power you are plugged into.

If the voltages are correct on the input side of the contactor, but low on the output side, then the contactor is defective. It is possible to replace the contactor, but most folks find it easier to replace the entire ATS.

Keep in mind you are testing hot circuits with enough oomph to harm you.

In addition to what Richard said, i always wear a pair of good quality exam gloves and tennis shoes. You can accidentally touch a 220v hot circuit and not get hurt.