Hi
Sounds like a wiring problem that only causes a problem when on the battery. The alternator on your engine is powerful and probably hides a wiring problem.
What voltage do you get when the engine is running?
1t should be around 14.4 volts.
Your charged leisure battery should be 12.8 volts.
What voltage do you get on the leisure battery when you turn on your USB panel? Does it drop when you switch on the panel?