I have always tested my interior space by using thermometers and small water bottles placed in different areas.
My 2018 Sunseeker 2350 also has the FW tank under the rear corner bed which is heated by the interior cabin space but also houses a heat duct that passes through the area under the bed which adds warmth.
The water pump is accessible inside an exterior baggage compartment which has a dedicated heat duct running to it to directly heat the pump. That outside compartment could bake a cake.
My interior floor has 3 levels , the cab seating area is the lowest, then the next level up is the kitchen area then the rear bed and bath area is raised another 4". The raised rear area seems to be heated because the ductwork and furnace are on the passenger side and heat travels across the unit to the drivers side bathroom sink which is equally as warm as the main cabin. The bathroom sink lines run through that floor. I have digital thermometers under the bathroom sink as well as the kitchen sink and those areas are heated protecting those lines .
My Arctic Package uses heat pads on the black and gray tanks and in MH's they are usually 110v pads due to onboard generators. TT's use 12v pads more commonly.
The rear shower does NOT have the shower head mounted on an exterior wall. My rear corner shower has a heat duct but also has the shower head and lines running through the interior heated wall next the bed. Those lines are protected by interior heat not only because they run on an interior wall but because the bottom of that wall is a gap that's open to allow the under the bed heat to heat that wall protecting that plumbing.
We don't run our class c on snow covered roads as we used to run our TC so we wind things up before the snow falls. All this extends our late fall early spring season down to 20F so far but during the dead of winter we winterize and use the rv as a dry unit. We hand pour windshield washer fluid to flush the toilet and hand sanitizer during our winter dry road day trips.
P.S. Because we use our rv for day trips, we leave an almost full gallon of bottled water under the kitchen sink for tea and coffee making. That water which is left inside the unheated rv 24/7 will only freeze when outside dip below 20F or so. During a mild winter like this year it only froze once. When it does freeze, it has never cracked the bottle, been doing this leaving a water bottle inside for 40 years .
