What is up with my brake lights???

in your brake light there a two filaments is on of them broken

nope, they are both fine

when the headlights and taillights are on, the brake lights are stuck on. when everything's off, the brake lights won't come on

When the brake lights are stuck on, headlights on, are the taillights also on?

I had mis-worded that. I am thinking some crossed wires...this really is confusing.

Turn signals work ok? Or are they a separate fixture on the RV body?

turn signals work okay, they are on the same fixture. i am thinking crossed wires of some sort too... been delaying following wires through the whole body. i'd rather not do that

Don't blame you on that. Time consuming and need patience. I just can't think of anything it could be, except crossed wires, one touching another that should not be, something that when you find it, it's a DOH! moment

is it a gound issue? my t100 would have tail lights, left would go out when light switch on and brake pedal applied. right worked fine, left would illuminate if tail lights weren't on (dimly) found broken ground at lamp socket. good luck, sounds like test light and meter time.