griz272, We can’t speak for any parks other than Yellowstone, but... there are a couple of National Forest campgrounds east of the park ( between Wapiti and the East Gate) that can handle a 42 footer. Live there and truck in/out of the park. Adds a few miles,but.... it’s still pretty and most of the critters don’t know where the park boundaries are!
We went from a 34 footer to a 41’7”” ( pretty close to 42’). Yes...it can be a rather steep learning curve. But kinda like in carpentry...”measure twice and cut once”! Just go “slow and easy”, and don’t get too comfortable....that’s when you make mistakes. Don’t ask me how I know! Don’t know the weight on your 42, but I suspect it was lighter than than ours (21,980#). With a newer diesel, it will probably tow pretty well!