I would go with something in the low 20's for length. Slideout is fine but the extra wide models are not. Anything bigger and you won't be able to fit in most of the campgrounds (might get lucky and find the 1 or 2 spots open in each campground that could kinda-sorta fit a 30' trailer, but usually not). A hybrid type (fold out bunks) may be a good idea for extra floor space (and they're light too), although some places like Glacier Park require hard sided trailers where there are bears. Make sure it's set up well for dry camping since you'll hardly ever have hookups -- LED lights, extra battery/generator/solar, way to take on and empty water tanks...
However, DO IT! The NP and Forest Service campgrounds are all beautiful and relaxing. So many people seem to HAVE to have hookups and space for their 35 foot toy hauler or class A these days, and they're missing out. Get off the beaten path.
