If it was previously a hot tub circuit with a GFI breaker in the panel, that *might* explain it but OP says it also happens at a CG and at a friend's house on 110. And then it is 100% okay at the dealer. Maybe the dealer really didn't test the heater or GEN breakers? Maybe all the dealer did is replace the converter panel, charged $$ and called it a day without thoroughly testing it all? Doesn't quite add up yet.
Shut off the heater element at the HW heater or remove the element and see if it still trips. What is connected to the GEN circuit - misc. receptacles and maybe a GFI one? If you were to pull the element out and the breaker still trips, then you'd know it's related to the wiring or disconnect switch at the heater.
Maybe try turning off all breakers in the converter panel (except 30A main) and try turning on just the heater or GEN breakers and see what happens.
If you have an ohmmeter, shut off all the breakers inside your unit and disconnect the shore power cord. At the converter panel, check resistance of GEN and heater circuits - L-N, L-G & N-G.
It could be a faulty heater element and short in the GEN circuit somewhere. If it is a recept. in the GEN circuit causing a short, you would have to pull each non-operating receptacle out of the wall and inspect the connections on the rear of them. Worst case scenario would be a short in romex cable somewhere. It does *seem* it could be a GFI related or maybe grounding/bonding type issue.
Having seen how they ram screws in any old place without regard for wiring in walls and ceilings, it's not impossible for a screw to have shorted out wiring.