You will be fine...I don't think you need to drain the water heater, granted one side of it is exposed but it is insulated and sits inside the trailer. We have been out with ours in temps getting down to 27-28 and had no issues. Wise to disconnect your freshwater supply...
Thing to remember is that when you are risk of frost for a short time like that, then temp normally only hits that frost point shortly before sunrise and only stays there a cpl hrs.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.