Well, I found a way that seems to work quite well. I did not want to touch any of the furnace
wiring for fear of causing problems in operation, and I don't want to become a self-taught RV furnace
tech and have my life depend on it!
The key part is a "make on temperature rise" switch, like those that turn on blowers in kick-space heaters or fireplaces. The one I happened to have turns on at 130F, off at 110F. It is mounted with a spring and wire on the end of the furnace vent pipe, near the outside wall. When the furnace warms up, the fans turn on. It takes about 1 - 2 minutes after the furnace started. When the thermostat turns the furnace off, it takes about 5-6 minutes for the vent pipe to cool enough for the fans to turn off. The wiring goes through the cabinetry on the port side of the Scamp
. I can post pictures of the setup if anybody is interested. I tested it a few times yesterday and today and I hope my DW likes it if we go camping in "spring" again.