I'd want to have a remote reading thermometer in with the tanks to monitor temps. There might be access through a closet or storage compartment. Often the water pump is under the steps to bedroom in 5ers. I'd also investigate routing of water lines. Make sure they are all inside the insulated shell of the RV, not routed through unheated spaces. An open cabinet door or closet can aide in getting heat into spaces that need it.
Insulating a space that has no heat will do little to keep temps up, you actually need to add heat with a light bulb before insulation would do any good.
Also, many RVs use a duct from the LP gas furnace to heat wet bay areas. If you use a space heater to cut down on gas use you can allow temperatures to drop in the wet bay or underside enclosure. Again the remote thermometer can tell you what's happening and if you need to add a drop light with an incandescent bulb to give area a little heat.