A couple of additional comments:
We have a "4 season" 5th wheel with tanks and pipes above the plastic bottom, I dropped the plastic to fix a FW water leak, all there is between the tank and the outside is a thin layer of aluminum and the plastic underbelly cover itself. There are 12V tank heaters (I think they are blankets) available on new RVs now, they would do a great job of heating the tank bottoms.
This is a link to a manufacturer, who says "retrofitting is easy"
HeatersRegarding running a genny in the pickup box, our dealer said folks do this to run the A/C unit(s) in transit, I'm sure it would work in the cold, make sure that you have good ventilation, I have a Honda 3000 in the front of the pick up box but no box cover. People have put in a plug at the front of the RV to do this with a auto change over electrical box.
