RED should be Battery POS
White should be battery NEG
Trace White from battery to frame...verify it is ground/negative.
Then trace RED from battery to the in-line main fuse (or DC circuit breaker)....probably a 30A size.
Should be on trailer tongue......circuit breaker looks like a small box with 2 studs. May have a reset button ---small tab.
In-line fuse or tripped CB -----no power to/from battery
Then check inside at the converter DC Dist Panel......for a couple of 'reverse polarity' fuses. Probably 30A also. If blown no power to/from battery.