I've been doing some winter camping this year. The only issue I've had was the heat exchanger for the hot water system freezing. I decided to relocate the exchanger under the sink where it will not freeze while camping.
I found these nifty bulkhead fittings in 5/8" (heater hose size) to run the heater hose into the van without having to enlarge the existing holes.
The hardest part of the job was dealing with the 1/2" PEX tubing, but I ended up liking the stuff after buying the proper crimping tool for it.
I originally planned to have a three-way valve to bypass the heat exchanger in case I didn't want a hot thing under the sink during the summer. We'll see how it goes without before adding even more complexity. I also ran the heater hoses around the gray water drain to see if that will melt it when driving so I can drain the tank.