The space heater idea works, but you are talking about boondocking, so that means running the generator all the time. If you have electrical plug in at your friends place the space heater may work just fine. As long as you're heating the interior of the motorhome, the exposed piping in the basement area and your holding tanks shouldn't freeze unless it gets really really cold. Heat will radiate from the motorhome into the basement so I'm thinking you're good like that down to temps in the teens. If you're talking 20 below then there's things you have to do.
I don't winterize my motorhome at all in California. I have a space heater with a freeze protect thermostat that kicks on at 40 degrees. I keep that inside the motorhome. I don't do anything to my storage tanks or plumbing in the basement. We get temps into the low to mid 20's and I've never had a problem.