To diagnose you need one of two things,
A test lamp, or a volt meter
here is what I;d do..
I would buld a test lamp.. You can get a rubber lamp socket with leads at most any hardware or electrical store including Lowes and Home Depot.
Put a lamp in it, small say 15 watt
Now find your automatic transfer switch, Depending on your RV (30 or 50 amp) it will have wires inside labeled Generator L-1 L-2 and Neutral (Ignore grounds)
connect between a the Generator Neutral and either L-1 or L-2.
Start the genny, light should, quickly.
When the switch transfers light should remain lit
When power cuts out the light will be diagnostic
ON- it's the transfer switch
Off. the generator.
Shut the genny off as that's as far as this post goes.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times