Hope that solves your problem...
Your truck camper like my OFF-ROAD POPUP trailer probably gets used alot late in the season where you may run into freezing temps at night...
This was one of the main reason I went with the blow out method to quickly winterize my small foot print water lines...
When camping off the grid I just simply drain the fresh water tank and open the hot water water spigot while doing this and it also purges the hot water tank as well just in case there may some cold water left in it from when camping at hookup locations..
Then I just simply connect my tankless 12VDC compressor to the city water connector and blow the lines out to the sink. Then I will add alittle bit of the pink stuff to the sink p-trap. There may still be some water in the bottom of the hot water tank but it has alot of room to expand so has not been an issue with me...
My tankless 12VDC air compressor takes awhile to build up pressure so it is very easy for me to start it up and walk inside and open the sink spigots before doing any damage to the lines.
I can do this this rather quick and the next morning when it starts warming up again all I have to do is add water to the fresh water tank and back in business again...
When I finally quit camping for the season I just do the same thing where it sits at home until spring or anytime we get the urge to go somewhere in it before spring... I always check my hot water heater superficial rod at the end of the camping season...
The small floorplan campers certainly don't have all the long lines like a TT or bigger trailer will have... Kinda never liked the idea of flooding them with the pink stuff... We don't cook or drink from the fresh water tank or city water anyway - always use bottled water for that...
Never have had any freezing problems using the blow-out method starting around the late 2008 time frame when I got my OFF-ROAD POPUP camper...
Roy Ken