This question is specific to the hot water tank in an RV. In my case it is 6 gallons, propane heated and insulated with the only exterior contact the access panel, as seen in most standard RV's in the US.
Hard as it is to believe hot water can actually freeze faster than cold water. How it happens is not fully understood, but undeniable it does happen. See related posts on physics.stackexchange What is the status of Mpemba effect investigations? & Hot water freezing faster than cold water
Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real.
The RV is currently winterized and located outside in temperatures that are freezing and below. I am leaving from a frozen area, traveling hundreds of miles through freezing areas, and arriving in a warm area (Florida USA). I must flush the fresh water system and top off before departure. Which means my hot water tank will be full. I can either leave it unheated with cold tap water (from my home, temp unknown but not ice) OR I can start the water heater and bring it up to temp before leaving.
All the plumbing and the fresh water holding tank are inside the RV. We will be stopping occasionally and heating the inside of the RV, so no concerns about the pipes or fresh water tank.
Driving with propane heating on (air or water) is possibly illegal and definitely unsafe.
Should I travel through freezing areas with hot or cold water in my RV hot water tank?