You are E TX. Unless you spend time at higher elevations you might be a little, or a lot, uncomfortable. 8k isn't that high but in September, in RMP, the weather can change quickly. We had a mountain home at 9k ft. In July it would be low 70s and cool to upper 40s to 50s. Seemed nice to us buy my mother's teeth would chatter she would be so cold (from TX).
Been thru RMNP a few times but not stayed in it...went down the other side to Grand Lake. On one of the trips thru, in summer, they were wearing parkas.
IMO I wouldn't run open propane heaters in my RV. Get some blankets and long johns.