David,
I'm certainly not going to suggest you are wrong! Our motorhomes could be wired differently. I'm at the condo, so I can't double check the panels or wiring diagram. But, I think if I plugged in two space heaters into convenient outlets in our motorhome I would overload the single breaker for that circuit. It is not the inverter limitation, it is the limitation of the 120v circuit itself.
Part of the challenge is that, even with adequate shore power, it is divided into two circuits of 50 amps. Our genset supplies only 30 amps to each circuit. If our theoretical cold weather HydroHot failure is in the mountains, the genset might not produce full output.
As near as I can tell, the inverter "pass through" is 30 amps based on the breaker panels in the back. The primary panel feeds a 30 amp sub panel. The subpanel has three circuits with 15 amp breakers: entertainment, microwave and "everything else". The everything else circuit has all the easy to access outlets and 120 lights on it. In other words, I have a single 15 amp or 1800 watt outlet circuit. The rest of the available electricity goes other places besides the convenient outlets. I goes to places like the block heater, the air conditioners, the refrigerator, the washer/dryer, the very limited electric part of the HydroHot and the microwave.
(On a side note, the electric element built into the Hydro/AquaHot uses about the same amount of electricity as a single space heater on high. But, the heat it produces has to keep the Hydro/AquaHot warm and is distributed with liquid through tubes to heat exchangers hidden behind cabinetry. So even though it uses about the same as a single space heater, the heat and comfort you get is far less due to ineffecient heat loss.)
Typical space heaters and hair dryers draw 8 amps on "low" and 12 amps on "high". In other words, you can run only one on a standard 15 amp household circuit set to high with a 3 amp "safety" margin. You might run two on low but it is past the theoretical limit of the circuit.
Our most used motorhome heat source in "normal" weather is a baseboard style space heater in the living room. It is set to low (8 amps) and we know where to set the knob to maintain about 68 to 70 degrees. It works fairly well down to about 50 degrees F outside.
In "abnormally cold" weather, the baseboard heater does not keep up. 8 amps turned into BTUs is not enough. I think that is about 3000 BTUs. I recall the HydroHot is capable of 50,000 BTUs.
My suggestion of using space heaters as a HydroHot backup is aimed at solving the problem of being in very frozen circumstances with complete HydroHot failure. In other words, somewhere below 25 F outside air temperature! I would want heat in my tank bay at 8 amps, basement storage at 8 amps, perhaps the HydroHot itself to prevent freeze cracking at 8 amps, bedroom at 12 amps and living room at 12 amps. That's 48 amps total.
I've installed 4 additional "utility box" outlets in circuits that do not use the "everything else" circuit. In my emergency plan, that circuit will heat the tank bay and basement. The additional utility box outlets in each air conditioner circuit power the two living space heaters. The electric element in the Hydrohot should keep it from freezing and provide some heat if it is only a diesel burner failure.
I freely admit that my freezing weather/HydroHot failure plan is complex and the best solution is the same as always shows up in the winterizing threads --- stay south in the winter! However, our South Dakota blizzard lasted three days. By the second day, the wiring diagrams started to become interesting! The screeching noise in the diesel burner was growing louder. At the time, I had only one space heater. At that moment, I wanted a backup plan! My heaters and utility box outlets cost about $100. On my motorhome, there was easy access so it only took a few hours to put them in.
In summary David, I think you could plug in one space heater in the basement on low. Another into a convenience outlet inside, also on low. A third, in a convenience outlet should trip the breaker UNLESS you have two convenience circuits with their own breaker. You may be able to plug one into the microwave outlet and another into the washer/dryer outlet -- if you can get to it. You would certainly get by down to about 25 degrees outside as long as you can keep the Hydro/AquaHot from freezing. I have a smaller genset than you and would run into the 30 amp limits to each leg.
