Your dealer is correct. The propane furnace has small ducts that heat the water pump/filter compartment and sewer connection compartment, as well as the black and grey holding tanks and pipes. On our Sunseeker 3100SS, the fresh water tank is under the bed and it will also stay nice and warm.
I put a remote temperature sensor in the water pump compartment so I can keep an eye on the temperature. We once were in 19 degrees F fully exposed to a 10 - 20 mph wind all night and the compartments never got below 55 F. Of course, we were keeping the coach about 70 F since I get into real trouble if I let my dearly beloved's little dog get cold.
A point to consider is that although a portable electric heater may keep the coach warm, it doesn't circulate warm air to the tanks. Use an electric heater as you like, but keep an eye on the compartment temperatures.
Keep the water heater on. The propane flue lets cold air circulate through the water, just the same as it heats the water when you have a flame. Switch it to propane if you need to manage your electrical load, like if you are running an electric heater.
If you have a heat pump, it will keep the coach warm, but since the ducts are in the ceiling, the floor will get cold. I sometimes run an oscillating electric heater on low to help that. Below 40 F the heat pump periodically defrosts, so you lose some heating time. Around 32 F our thermostat switches to the propane furnace even if the heat pump is selected. I usually switch to the furnace at about 45 F.
In freezing weather I disconnect the city water hose and run on the fresh water tank. If you have the sewer hose connected, be sure it has drained well. I just put it away.
I hang a blanket from the overhead bunk to block off the cab. It's not insulated and you'll loose a lot of heat there. Check under your slides. Our 3100 has a gap in the lower seal, so I put door draft blockers on the floor to keep out drafts. You slides may seal better. I made window covers from Reflectix for hot and cold weather. I'd recommend that you make some when you have time.
2011 Sunseeker 3100
2016 Ford Edge Toad