Hi,
You have done a great job on the mods.
May I suggest adding a thermostat to the fan which was added for keeping the water pump unfrozen?
I replaced the return air grill on the furnace with a fan. On my rv that circulates warm "living space" air under the cabinets and pushes some air along the furnace duct work. I chose to use a dual window fan unit which is controlled by a mechanical thermostat that lives beside the water pump next to the outside wall of the RV.
The fan unit draws just 23 watts and works better than a 1500 watt fan based heater which I tried using inside the cabinet.

This allows me to heat 100% electrically.