For a couple of hours at a degree or two below freezing I wouldn't worry about it, plus a portable ceramic heater wouldn't do much anyway but keep the pipes inside the rig from freezing, water lines below TT require running furnace to keep them from freezing. We camp often in lower temps without issue.
For peace of mind and in case weather report is inaccurate, dump water from tanks including water heater and open low point drains. Of course you could go all out and blow out lines followed with antifreeze, again with those temps I wouldn't bother and I have an exposed underbelly on my rig.