RV's come in three configurations depending on the model. NOTE this applies to Motor Homes, all classes, as well as all types of trailers as well
Basic:
Shore cord--(Option)---Main breaker box.. Branch breakers to everything
The (Option) is a detachable cord and inlet,,, Most 50 amp RV's have detachable cords, 30's often do not.
NOTE: This option applies to all types of RV's.
Poor Man's Trnasfer Switch (This is for an RV with an on board generator)
Shore cord to everything,, Same as above]
Generator to an outlet.. To use Generator you plug shore cord into outlet.
Most modern RV's with Generator
Shore cord -- (Option)-- Auto transfer switch---Main breaker box and so on
Generator---------------- Auto transfer switch
Your suspect list
Shore power: Are you plugged into a LIVE outlet (Test it)
Shore cord outlet (Screws loose inside. I've seen it and I'm not an RV service techinician) (This is the option)
Shore cord INLET. Same as outlet (This is the part mounted on the RV
Auto Transfer switch.. Loose connections (have yet to see that but have read about it) Bad ATS (have seen
Breaker box Loose screws (have seen) Bad breakers (Have seen) Other problem (Have seen) Inclucluding a breaker (main in this case) not fully seated.
Broken wire. (Anywhere between plug and breaker box. and yes, I just replaced a plug 3 weeks ago.
Shore cord (Option)