The heating element could be shorted out inside the threaded end. You'd never see it since it's sealed.
Is it possible you are running other electronics inside the camper that is pulling a lot of electric draw? Are you running the air conditioner and microwave the same time you are flipping on the water heater?
Considering your Main is blowing, and not the dedicated one for the water heater tells me, there's too much electric draw. Simply, everything combined is near 30 amps. When you flip on the water heater, you've just exceeded that 30 amp breaker and it pops!
Turn EVERYTHING off inside the camper. Then turn the electric on the water heater. See if if pops then. If it does not pop, then turn on one electric device at a time and you'll soon learn the campers electric limitations. Not everything can be on at the same time.