On the way back from our Big Bend trip last week, we had no choice but to drive through the frigid temps in central TX and AR, and no chance of winterizing before doing so. Our solution was to run the furnace while travelling, which probably isn't advisable but did the trick for the curbside plumbing nonetheless, with the exception of the bath drain traps which began to freeze but fortunately didn't freeze solid. Had I propped open the bath door somehow, that probably wouldn't have happened.
Our Truma is throwing a high current error code when on eco mode, so we had to drain that, but that's an easy step. I'll have to get Oliver to check that out - not thrilled at the idea that we might need our third Truma in two trips.
The street side back corner is definitely the trouble spot. We drained the outside shower hose and water ports, but the mixer valve on the shower froze, and there was enough residual water in the intake ports to collect at the bottom with a little plug of ice in each that had to be melted down before we could winterize once back home. Sticking our Vornado heater in the basement for a few hours unfroze those lines no problem.
I had Oliver install a 120 in the basement for a little marine compartment heater, for just this reason - unfortunately, that's still something on the list to buy. But I think a better solution, or at least a good supplement, will be to vent that whole area to the furnace side. I'm going to get three decent sized return air vents from Lowes and install one on each of the divider walls in the basement. That should get some heat from the furnace into the basement and then through to the outside shower connection. Then I'll install one on the floor over the water inlet lines to allow some heat down there. I probably won't get a ton of airflow, but it's a cheap and easy mod and any warm air is better than none. I can run the compartment heater when plugged in, or off the inverter in a pinch, and that should really heat up that whole space.