One (just one of many) of the things that I think is un-smart of RVIA is that they didn't look at either USCG or boat builders guidelines. As a result, RVs have a great deal more electrical trouble than they should. In the time before the current depression, I did a great deal of boat work, but I also got roped (well, they did pay me) into tracking down electrical issues in RVs. Loose connections is a chronic issue. That is exactly what you are describing.
Use a test light and not a meter.
Check both sides of every switch and connection.
When you can't tell where a circuit runs, get a tracer. (A tracer is a gadget you put on the line and it uses a radio signal that an included receiver picks up.) It may not be cheap (the cheap ones still do work), but think of what it will cost to have someone like me find the problem.
There is no quick answer, it takes the doing to fix these.
Matt