Tom,
The electric portion of a HydroHot uses a heating coil identical to the ones used in domestic hot water heaters. It is rated for 1400 watts, plus or minus a 100, depending on model. That is the same as a consumer space heater on high. If I remember right it is about 5,000 BTUs. The diesel burner is about 50,000 BTUs.
My rule of thumb is that the electric is enough for raising the temperature about 15 or 20 degrees. If it is 50 outside, I'll maintain close to 65 inside. I'll guess that if it is 0 outside, the electric only might keep your motorhome interior at about 15.
Since most "camp" in mild climates, the electric works pretty well and keeps the noise and smell of the diesel minimized. But if it is "cold" there is not nearly enough capacity in the electric only system.
Regarding price, I once looked up some complicated charts provided by the Department of Energy. Generally, electric is cheaper than diesel heating oil, but it depends on the price of electricity.
My fear using electric only is if there is a power failure or the element burns out. I've had both. If it is below freezing and there is no heat inside the HydroHot itself, it will freeze. The repair is about an $8000 replacement unit. The ONLY secure way to protect your HydroHot in cold climates is to pump it full of pink RV antifreeze.
About the Xantrex. It is not hooked to 50 amp 220. In our motorhomes, the 220 is split into two 120 systems. The Xantrex is on one of them. In other words, it is actually wired to a single 50 amp circuit. Off the 50 amp circuit, the Xantrex is feed through a 30 amp breaker. All the Xantrex will ever get is 30amps of 120. There is no reason for settings any higher.
The only time the Xantrex will draw close to 30 amps is if you plug in with nearly dead batteries. It will work as hard as it can to charge them, and on mine, can consume around 25 amps for short while and then taper off. Each AC uses about 10 or 12 amps with a short peak at start up. Each is wired to one of the two 50 amp circuits. So when you are plugged in to "50 Amp" power there is more than enough to run both A/Cs and the Xantrex.
Call, write or email if you have questions.