Friends on a couple of RV women's forums suggested many ways to get rid of the critter or critters, such as attaching fabric softener sheets underneath the rig, stuffing steel wool into every possible opening, using sticky traps or regular mouse traps, and so forth. They told me to do something quickly because mice and squirrels love to chew through things like water lines. But, first I'd have to get underneath and try to determine how the things were getting inside. It was about 32 degrees outside, way too cold to go crawling around under an RV, especially with 30 MPH winds blowing. So, I did the next best thing---I took time out from the hunt to check email.
That turned out to be probably the best thing I could have done because my friend Yarntangler wrote that there were three vacant sites in their part of the campground, sites with full hookups and a lot less wildlife. And so I did my turtle imitation: I packed up as quickly as possible and drove over there. I pulled in between two much larger rigs, which should help block the wind a little, hooked everything back up, and am now comfortably at home--with no more strange rustling noises. Hopefully the creature, whatever it was, moved or fell out during the five-minute drive.
As much as I complain about the cold here, the expense of two vehicles, and the constant upkeep required, I've got to admit one thing: in all my years of living in "stick" houses and apartments, I was never able to just pick up and move in 15 minutes. I won't live this way forever because I'm already dreaming of having another garden, of collecting more books, of buying a piano, and of putting them all into a small house of my own somewhere. But for now, this is home and I love it.