So, I bought a 2005 Sportsmobile EB50 over Christmas/New Years and took it camping for a week or so in Death Valley. It got below freezing overnight, which taught me a few things:
1) You have to open the drain valve under the city water inlet connection to drain the water out of the PEX pipe that runs under the van (this should, theoretically, be the only piping outside of the insulation).
2) You then have to close this valve. Otherwise, when you turn the pump on, it'll pressurize the system, send water back down this pipe, and shooting out the drain valve.
3) Which means that you have to (annoyingly) get in and out of the van every time you want to run the faucet (so that you can close the drain valve, run the water, then re-open the valve to drain the water again).
I don't see myself ever leaving my Sportsmobile "plugged in" to city water -- so this is strictly a fill port for me, which is a purpose also served by the internal fill port. Since one of my primary goals of owning a Sportsmobile is winter camping, I plan to use the interior fill port almost exclusively.
So, in order to keep the drain valve open (and the exterior PEX pipes clear), I decided to install a ball valve right where the PEX pipe enters the van from the bottom. This way, if the ball valve is closed and I turn on the water pump, water won't go shooting back out the drain valve.
Here are some pictures --
Step 1: gather supplies.
From left to right -- 1/2" Sharkbite push-to-fit elbow joint, Milwaukee pipe cutter, 1/2" sharkbite push-to-fit ball valve, wire cutter, Sharkbite deburring tool, Sharpie, and some zipties.
Step 2: disconnect the gray tank. This connection is on the passenger side rear wheel well.
Step 3: drop the gray tank, which is just rear of the rear axle.
Step 4: locate where the PEX pipe from the city water inlet port enters the van (mine was just to the rear of the gray tank, right above a crossmember)
In the pic below, it's the long white tube that enters the van. The stubby tube next to it (poking out from the van bottom, pointed straight down) is to drain the fresh water tanks.
Here's another pic of it, for some frame of reference
And a close-up
Step 5: cut the PEX tubing about 3" from where it enters the van
Step 6: push on the push-to-fit elbow and ball valve
Step 7: reinstall gray tank
In the image below, you can now see the ball valve sitting above the crossmember. In the summer, I can just crawl underneath, and open up this ball valve when there is no danger of freezing.
Then you're DONE! Pretty easy mod. Only non-ideal thing is that there is obviously still a ~6" length of PEX pipe/elbow/ball valve where water can (and will) freeze. Ideally, the ball valve would be installed inside -- but the way my plumbing is on the inside didn't have space without a major re-routing of all the plumbing. I'll probably do that in the near future, but this is a stop-gap measure that should get me through at least this winter.
I'm also going to add ball valves to 'lock out' the cold and hot water heading over the back door to the shower (interior and exterior). That's an area where a bunch of water currently gets wasted due to pipe fill, and can be one less freeze area if I keep it dry.