How cold you can go in your RV really depends on how it's built. Even with the trailers I had that had exposed underbellies etc. I could generally get by without issues when temps got down to around 28f at night. Extended periods of cold they would freeze up even at that. The worst I had happen was my gray water drain freezing and cracking when the night temperatures were getting down around 15f at night.
With the Timber ridge I had prior to my fifth wheel, and with my current bighorn I haven't had any issues with nighttime temps getting down to single digits at night. I just set the furnace to 55f or so, put a heavy comforter on the bed, and turn up the heat in the am at the same time I light the stove for the percolator. It's nice and warm inside by time the coffee is ready.
As one who camps in the areas you're looking at a lot in the shoulder seasons, I doubt you will have any issues unless you have water lines completely exposed without any protection whatsoever as long as it's only a few hours you getting down into the 20's at night. It's a combination of temperature, and exposure time to the temperature that freeze things up.
Have a great trip.