I don’t know what sort of RV you have, but since you prefer FHU’s I’ll guess it’s a larger one. And they don’t lend themselves well to impromptu stops. Commercial parks are you best bet. I have a little 16’ and it’s a rare event that I can’t get into the popular park campgrounds even in prime time. The HU’s are your problem, dry camping you’ll have a great many options even if the size imposes some limitations on you. And be aware FHU campgrounds in the parks are limited; there is only one campground in Yellowstone and the Tetons that have hookups. Stopping early, avoiding weekends and prime time will all give you an edge on finding a spot. But by far, willingness to dry camp will maximize your options.
