Hmmm...so there's maybe about a 9V drop on just that circuit when using the heater.
I would strongly suggest unplugging the RV temporarily, opening the breaker box (and the inner cover so you can see all the wires), and checking that all the screws are snug and the wires do not show signs of overheating. It seems like there is excess resistance somewhere on at least one of the circuits, and that generally means a bad connection or bad wire somewhere, and heating due to the power the poor connection or conductor dissipates.
If the heater is using about 12A (roughly typical on high), and the voltage drop is at a single connection, then there is around 100W being dissipated at that point. That's a lot of localized heating, as anyone who has touched a hot 100W light bulb knows.
