I have a '14 37TS and I believe the tank heating method is the same on the newer Senecas. The tanks are indeed heated by the furnace, not electric tank pads like some other Jayco motorhomes. So in sub-freezing weather it is important that you use the propane furnace so warm air is circulated to the basement area(s). Above freezing it is acceptable to use electric supplemental heaters (rather than burn propane) but if it is below freezing you need to move some heat down.
My heat pumps do indeed work to about 40 degrees outside. Colder than that the compressor shuts down and there is logic that will fire the furnace if the front thermostat isn't satisfied by the heat pump.
Now the water heater is a whole other topic. Multiple threads on these units on this forum. Some love them, some hate them. I was in between, mine worked fine if I was on full hookups and could afford to "waste" water. Generally you have to maintain steady water flow continuously during your shower or the temperature fluctuates significantly. No "Navy" or "GI" showers! Since I dry camp frequently I needed a water heater that allowed a trickle flow without shutting down on the high-limit safety then giving you a cold water blast until it reset and warmed back up. So very recently I changed my water heater to a 10-gallon Atwood XT tank-style unit. I did the work myself, all that is left is to get the new exterior door painted to match my unit (see below).
Overall I love my Seneca, it is our third Class C and it drives like a dream. Better fuel mileage than my former Ford Class C units and you don't even know you are towing a car!
Please feel free to post any questions, chances are one of us here has already "been there, done that"!
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST