A few questions , I assume your water holding tank and sewer tanks are heated?
If so then buy the blue or pink stray foam board and box in tanks , the sell silicone to glue the seams or use wood screws
Next heat trace water lines and insulate with 3/4"
Foam pipe insulation
Buy thin plywood and screw the foam board to plywood and skirt the trailer
On the inside open cabinet doors where there are any plumbing lines so they will get heat
Your sewer should be fine , hook up lateral line to sewer connect , keep valves open
We live in Texas also , where are you going to be ?
2009 Rambox crew cab
2012 831 Flagstaff RKSS
2010 jeep Liberty
2010 Harley Davidson Ultra Classic