3 days is no where near enough for Thermopolis AND Cody!
If you enjoy hot pools, you will want a couple days in Thermop.
If you enjoy Western art and Buffalo Bill and firearms exhibits, you can spend 2 or three days just at the Buffalo Bill Historical Center in Cody.
I have been to one of the RV parks in Thermop, and my 32 foot Class A fit just fine. I have no idea how well a rig in excess of 40 feet will do. You can call the park and ask if a big rig would be a good fit.
Remember, it is quite a ways from Thermop to Cody, on a 2 lane State highway.
I recommend that you find a place to park in or near Cody, spend your three days there, and then proceed on your trip.
You have a quite ambitious itinerary laid out. I do hope you are aware of the distances involved, just in Montana and Wyoming!