The "electric heater" is most likely your A/C running in Heat Pump mode. When you run an A/C unit backwards rather than take out heat from inside the RV it takes heat from the outside air around your RV and pumps that into your RV via the A/C ducts.
This is obviously totally separate from the propane heater in the floor.
Heat Pumps only work when outside air is approx 40 degree or higher. With lower outside temps around 37 to 40 there is not enough heat in the outside air to be effective. Many systems will automatically switch from Heat Pump to the Propane Heater when outside temps drop to ~40.
Some A/C Heat Pumps do have optional "heat strips" that can be installed in the airflow. These work like an electric heater and simply heat up the air being pumped into your RV. I don't think every A/C HP unit can be retrofitted with these - but surely some can. With the addition of these your HP will provide heat below 40 degrees. Most folks just switch to their Propane Heaters.
Generally, the Water Heater can be fixed to work better - and not needing to be replaced. There are tons of Atwood Service Locations to work on that and a number of YouTube videos if you want to work on it yourself. Yes, the coach coolant from the engine circulates in the heater. WBGO calls it Motorad, I believe. "Usually" the water heater with Motorad is not super easy to replace - between the coolant lines, propane lines, etc it's a complex device.
The motorad feature is great - on travel days you arrive with a full tank of hot water before you even turn on your water heater.
I don't know how you remove the water heater - I have heard of folks saying you have to do a lot of work disconnecting things on the back of the water heater and that can only be reached from under the RV.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad