If you're only worried about temps of 15 degrees and above, just adding heat pads to the tanks would go a long way toward fulfilling that need.
If I were to tackle this project I'd not only add the heating pads, I'd also add a heat trace cable to the water pump suction line which often runs through an unheated, cold, area. Same company that sells the pads sells a water line heat trace wire that also runs on 12V.
I'd also add insulation to every part of the underside of the trailer not already enclosed by "Coroplast". Find a company that sprays foam insulation and have them cover all the areas outside of the covered area, including dump lines out to the dump valves.
The rest of the task would include making window covers out of either Reflectix or foam insulation board. Cover all the windows you don't usually look out of regularly and have covers for the rest to put up at night.
Beware: you really don't want to seal an RV tight while sleeping in it. You do need SOME fresh air to breath.
FWIW, I lived through a winter in a 1995 Terry which only had a heater on the black tank. I was in a park and left the Gray tank valve wide open. City water hose was heated and as a backup I kept my Fresh Water tank full, It was under a jackknife couch above the trailer's floor so it was at no risk of freezing.
The underside of the trailer was merly covered with what I call "Landscaper anti-weed cloth" which was supposed to be rodent proof.
One morning the temp was down to -15 degrees and no water froze.
I did burn a lot of propane. The local propane destributor set a large horizontal tank at my pad and I hooked to it. They kept it full and were delivering propane every three weeks or so, Furnace ran all the time and my TT looked like a big Ice Cube with all the melt from the roof that re-froze on the sides. At the time propane was about a buck a gallon with the "keep full" program. I spent around $50 per month during the coldest months. Today it would be about three times as expensive.
The key is to add insulation where it's weakest, underneath and at windows.