Funny, my previous trailer was a 2104s and I was toying with the idea of heating the underbelly. My new trailer has the enclosed underbelly, tank heaters and the furnace vents in the floor so for the camping I'll probably ever do this should be fine.
I don't know if I would consider tank heaters expensive ($100's), especially compared to the cost of a diesel furnace ($1,000's).
I'm guessing that you want to do some serious cold weather camping. My thought would be to heat the underbelly by putting a small furnace vent or two in there (and some holes for returns I would guess). There are posts here where people have put foam board along the frame rails and put a 1-2" think foam board under the frame (covered by the corrugated plastic thats already there). Since the tanks and valves hang somewhat below the frame that makes it tough i.e. the surface is not flat so some custom cutting needed. Just want to seal it off as good as you can so your heat stays there.
I don't know if I would blow foam into the space. I don't know if you should have air flow around the frame, wire, etc,... to keep the moisture in check. Others probably know better than I do about this.
The black/grey valve etc.. you'll need some form of pipe tape/heater.
2020 Flagstaff Super Lite 26RBWS
Former: 2017 Rockwood MiniLite 2104S
2015 Silverado 2500HD 6.0L Gas