I have a 2017 Sunseeker on a Ford E450 so I'm not sure how that translates to the Transit series...but, My FW tank is above the floor and under the bed. The Gray and Black holding tanks are below the floor and mine has the tank heater package. There is no plumbing except the low point drains that are below the coach floor. The water pump is not in the coach per se but it is behind a cabinet and next to the water heater. Easy for me to open some cabinet doors and drawers and keep heat into those areas.
My MH does have Azdel walls. As far as insulation from the cold I probably can't help much! I'm done with cold and live where it is warm!
The coldest we had this winter was upper 20's F and I never winterize but run a small electric heater in the coach and open the drawers and doors mentioned above during our coldest snaps.
When we camped this spring in Oregon we ran the furnace at 62 F overnight and during 40 degree nights it cycled a few times towards morning.