We went from a PUP to a 22' HTT (hybrid trailer) to our current TT, which is considered a 31' but is 34' from hitch ball to spare tire. We found we had definitely sized ourselves out of some of our favorite SP campsites (Wyalusing is out of the question - one of our all-time faves; we even had trouble fitting the hybrid into it.) I think you will need to figure some kind of compromise between your space needs for longterm stays and your size needs for state parks, COE campgrounds and the like. I think the eastern parks especially tend to be older and therefore have smaller campsites; that said, I know some of the national forest CGs out here are also old, with 24' being a fairly common max size for trailers. 24' is probably doable for long stays, if you enjoy being outside or on the go a lot.
We have taken several 4- to 6-week trips with our TT and have loved the interior space (only one slide but it's almost half the length of the camper). It has been exceptionally comfortable for our lifestyle and has served very well as a second home. However, we miss being able to stay in some of the more wilderness-type sites (even backing it into some of those sites is a real challenge). We are seriously considering downsizing to something 26' or less - but then, we are also leaning toward shorter trips, perhaps 2-3 weeks in duration, so interior space is less important.
A thought: if you spend most of your time in more natural settings, you may likely spend more of it outdoors, and may not care as much about having a really spacious trailer. We have found that we spend much more time inside, with such a big trailer, and we'd like to change that.
We went from a tent to a small (18') MH to a PUP, which after many years we upgraded to a HTT and finally a TT. In general, we like having a trailer better than a MH, mostly to have camp set up and a vehicle free to sightsee, grocery shop, etc. - but w/o the absolute need for a pull-thru site that a MH+toad would require.
You'll probably be skimming the surface to see all 3 in 2 weeks time, but it's more doable from Portland than from the Midwest in that timeframe. Last week of August is a good time - but still in high season, so you WILL need reservations around YNP and probably Glacier as well. Fortunately you'll be a couple weeks after Sturgis for the Black Hills, but that doesn't mean campsites will be wide open.
I'd recommend spending at least 3 days in & around Yellowstone (3 nights as well) and probably the same for Glacier (though no personal experience at the latter). The Black Hills can easily fill a week. My suggestion: skip one of the 3 and just do 2 of those locations. Leave off the Black Hills or Glacier. You're likely to enjoy your time better at the remaining two.
Should you choose to do YNP and the Black Hills, routes between the two depend on your comfort level with mountain driving. It's quite a bit longer taking the northern route (I-90) between them than going more direct; all the info on here points to US 16 as by far the best choice across the Bighorn Mts if you do want to take the more direct route from Cody to Buffalo WY. Either way, a stop at Devils Tower is a good idea. Your MH will have a tough time finding a spot to park inside the park there, but it's worth an overnight at the KOA, just outside the park (fantastic view of the Tower from there!) If you do stop there, be sure to get up for sunrise.
How did it come out? I would guess your pipes would have been ok, but the hose was probably toast. We've camped there multiple times and usually end up using the tanks. Yellowstone almost never has warm overnight temps, and when it does, that usually means something very bad - as in '88, when it was warm enough for shirtsleeves overnight, but 1/3 of the park was burning!
Have done both, and IMO the scenery is somewhat better along US 26. It's a good road, and though you do go through towns, there aren't a lot of them. :) There are some interesting pioneer sites along that route, Chimney Rock and Scottsbluff the most prominent. Real nice CG in the Scottsbluff area, Robidoux RV Park in Gering NE, which is a city-run park - hookups, clean restrooms, and good view of Scotts Bluff from the CG. I recommend it. (DO get reservations.)
I would second the recommendation to stay in West Yellowstone and rent a car for the duration of your stay. There are so many times you will want (or need, due to traffic ahead of you) to pull off alongside the road, etc. and that is so much more easily done with a car than with an RV. Plus other drivers will appreciate your consideration! :) I recommend Grizzly RV Park - close to town and the west gate of Yellowstone, clean & nice, full hookups, etc. You may even be able to reserve a spot a year ahead there (though I have not tried it).
Does your TT door have a deadbolt (with key) in addition to the regular lock? If so, use it. The regular door locks are not individual to each trailer, so anyone with a key to another TT of same make could get in that way. Deadbolt locks areindividual to each trailer, however. At least, that's what our dealer told us. Whenever we left our TT there for service, he asked us to unlock the deadbolts but leave the other locks locked, since he had a master key & could get in if need be.
Another possibility is that someone is pretty good at picking locks. :(
Seems like a long way to go, by our driving standards. The Great River Road is not a fast way to travel - you do have to slow down for small towns (which can be fairly numerous) It's a delightful trip (have been only on parts of it in IL & WI) but not even close to interstate speed.
Ianmac, have a great trip! I'd advise making sure you fill up on gas before you get to 1/4 tank left, especially if you're going across Wyoming. Otherwise, as you have probably already experienced, it's plenty civilized. Well, enough, anyway. :)
Did you need help with routes, must-sees, etc. - or just wanted an "atta boy" from us? If the former, fill us in on how much time you plan to take for the trip and what your priorities are. Do you have reservations for Yellowstone? I sure hope so. You'll be there in prime tourist season.
Lots of great advice here. Like many others, I also say it's up to you. If you enjoy history, definitely don't miss Little Bighorn. Interestingly, tours will vary in emphasis, depending on which ranger gives the one you're on - don't miss their talks in the visitor center. You're guaranteed to learn something you didn't know, no matter who does the presentation. Devils Tower is amazing; if you haven't seen it, go. The KOA as a CG is decent enough, but you can't beat the scenery - it's right at the foot of the tower. Good spot for an overnight & visiting the tower.
As for leaving the park, the only flat way to leave is west entrance, and that may have been why you read about it. East entrance would be my second favorite, and going north from Cody to connect to I-90 is a good route, nothing steep, and pretty scenic.
A disadvantage of a fairgrounds park is if some event is going on there - then if you can get a site, you're cheek-to-jowl with other folks. If you're not keen on KOAs and don't need tons of amenities, just a clean decent place, Elkhart CG works well. I'd recommend reserving a spot - they're popular and if there are local events or rallies, you could end up in a muddy field. (Caution: most sites are on grass, not gravel.)