This is the approach we took to fix a water heater feed line and valve that failed. It was faster and easier to transition form cpvc to pex then back into the cpvc. We're planning a full re-plumbing process in the next year replacing all lines with pex.
Note the white fitting extending from the heater inlet is a backflow preventer and must be kept in place. The current way the system is plumbed and the presence of this valve requires that antifreeze be used to flush water from the fittings. Additionally, the existing piping needs to be revised to assure better drainage to drain points.
We also observe that to assure this valve and turn-off and piping points are not damaged, antifreeze must be pumped from the fresh water tank to the water heater until is exits from the water heater drain point (the 3/4" anode port), and exits all cold water plumbing points. A trick we do to assure that the antifreeze reaches the hot water controls and piping is to run the cold water taps until the antifreeze exits, then, with the hot and cold water taps open, place a finger over the spigot outlet to force the antifreeze back into the hot circuit. This has worked well in both the Fiberstream
and our Airstream.
Lowes had all the parts, and it was a quick, simple fix.
Currently, for cold weather camping, we have two 75 watt incandescent bulbs in work light
holders for keeping the underbunk area from freezing.