As someone who grew up in the Dakotas, you most certainly can't count on the temps staying in the mid-20s to 30s. When I passed through that area a few weeks back, the weather guessers insisted that the low was going to be 30... but when I arrived at the RV Park at 6:30 in the evening... right at the SD line, it was already 29... and it got down to 20. I did lose my water, but luckily the hoses did burst.
A cold front at this time of year can sneak in and keep the temps from getting even up to 30 for days on end... and you'd be in trouble. Personally I would winterize the rig and use jugs of water for everything... and have the pink stuff for toilet flushing if it gets really cold.
Kansas "might" be warmer...