Between SLC & Wyoming on I-80 there is a pretty good climb - depends on what you're trying to avoid. We had no problem with it whatsoever with our rig. You may be able to avoid some of that climb if you use I-84 from Ogden east to I-80, but you'll catch some of it.
Not sure Idaho will help much. We have mountains too. :)
If I remember right, Indian Lake SP (MI) isn't far off US 2. We have camped at both east & west sections of the park; west has only 20 amp hookups and no showers, but for one night in the fall of 2009 it was idyllic, very quiet and pretty (wooded, but sites plenty big enough for our rig.)
Welcome to the darkside! :) We also don't regret our move from HTT (previously had a 21SSO) and have enjoyed over 20,000 miles of travel with our TT since 2008. Timing turned out to be good, since DH retired sooner than expected. Enjoy your new rig!
A good choice for base camping for Yellowstone would be Grizzly RV Park in West Yellowstone. Nice clean CG with room for the larger rigs, FHUs, etc., and considering CG prices in the area, not terribly expensive. We have stayed there multiple times. Very convenient. If you don't have a toad, you may be able to rent a car in West Yellowstone for touring the park - much better than doing so in the 43' RV, though the latter is done all the time as well. Definitely plan several nights there. Get reservations for Yellowstone NOW.
Cowboy RV gets better ratings, but we have been to the Pocatello KOA several times & prefer the location - a little easier on/off from I-15, IMO, and though the park itself is old & a bit shabby, the scenery is better there than at Cowboy. For an overnight, it's fine, and has more sites available than Cowboy (from recent observation.
I second the above recommendation for the SLC KOA. For an urban CG, which it is, it has a surprisingly isolated feel, and many sites have some shade (a bonus in the summer!) I'd recommend asking for one of the premium sites if you're spending more than a night - they do have more space and are probably better for the longer rigs as well. They'll be closer to the facilities in the satellite building, which we found to be somewhat cleaner than those in the main building, due to the latter's proximity to the pool. Convenient to the downtown sites too (Temple Square, etc.)
Congrats! Hope you enjoy it. We have fond memories of our '05 Starcraft HTT (different model). IIRC, that was the first year they went to the truck-style latches on the bed ends, instead of screw-style ones, MUCH better.
I prefer GS over AAA. Do a side by side comparison to see if it's right for you.
I second that recommendation. Our experience in the past was that AAA did not cover RVs in their emergency road service; GS does. Also, AAA CG listings are sparse, and GS discounts on camping fees seem to be more available than AAA ones.
Three weeks is a pretty good timeframe for that itinerary. I'd suggest going via I-90 one direction, visiting the Badlands & Black Hills in the process. Devil's Tower isn't terribly far off I-90 in eastern Wyoming, and there's a decent enough KOA right by the tower, with a great view from many campsites. (1-2 nights there should be sufficient; 1 would be enough if you're going there right from the Black Hills.) Another good stop along I-90 in southern Montana is the Bighorn Battlefield National Monument (Custer's Last Stand). Worth a day, IMO.
You MUST have reservations in/near Yellowstone. CGs inside the park may be booked up by now, and only one (Fishing Bridge) has hookups. Decent CGs in Gardiner, MT (north entrance) and West Yellowstone (west entrance) may still have openings, but I wouldn't delay. I would suspect you will need reservations near Glacier as well. Remember that Glacier has a short season, compared to Yellowstone (and certainly compared to southern Indiana!) When were you planning to visit?
Along I-80, which would be a good way to/from SLC, there are a lot of pioneer-related sites, including "living history" villages, state parks, etc., in central Nebraska, which are really interesting if you are into history. Not especially cheap or simple to get to, but the big arch museum over I-80 in Kearney, Nebraska is worth the stop.
I second the must-see on the Tetons. Allow yourselves at least 4 full days in Yellowstone & the Tetons combined. (Tetons make a good day trip from Yellowstone; the parks are virtually contiguous.)
Between Yellowstone & SLC, the easiest route is via I-15, but a more scenic (and longer) route goes through Star Valley, Wyoming, by Bear Lake and through Logan Canyon. Worth it if you have the time. There's a nice little museum dedicated to the Oregon Trail in Montpelier, Idaho, along that route, and is worth a couple hours.
Not sure about reservations (check nps/gov/yellowstone for info on which CGs in the park take reservations) but I believe the smaller CGs in the park do not allow generators. The bigger ones do, but restrict the hours they can be run.
Unfortunately, Willard Bay SP near Brigham City is closed due to issues with the water supply. I don't think they have an estimated open-by date yet.
Note that Massacre Rocks will be east of where I-84 connects to I-86, so it will be out of your way. That said, it is indeed a nice place to camp if you don't have a big rig. Ours (see sig pic) fits in only a very few spots there, and we had to reserve one ahead of time to make sure we would fit.
Friends drove through Mountain Home RV Park and said it looked quite nice; haven't been there yet, so can't offer any firsthand info.
Even if not right on the water's edge, a site at Aune-Osborn is worth it, IMO. Next door (walkable) to a good burger joint and a local park along the water, and the CG has benches positioned to watch the river. We really enjoyed it.
Caveat - be careful with cellphone calls from there. They may get routed to a tower on the Canadian side & incur international rates.
Approx 26' feet long? Way too long for the Explorer's wheelbase, IMO. Had a 2002 Explorer with which we pulled a 23' HTT and even with good sway control, it was the tail wagging the dog. I'd recommend an excellent brake controller, BTW - you'd have enough engine for this tow, most likely, but Explorer brakes always left me a little concerned. Good luck with your decision.
Massacre Rocks is a nice SP but pretty far from IF. Depends on how close the OP needs to be to town. If 45 miles will serve, there's a KOA in Pocatello. Not the best CG but adequate & has nice scenery. Open all year. They've got plenty of sites this week (we drive by them frequently).
If you are good at marathon driving, you will probably make it to Toronto & back in 3 weeks, but you will see almost everything through the windshield. Some years ago, we pulled a popup from the Chicago area to the Washington coast & back, and we ended up spending no more than 1 night in any given place, except for about 2 or 3 places (where we spent 2 nights). That's a LOT of setting up and breaking camp (especially with a PUP, I'll admit).
That said, you've gotten a lot of good suggestions so far. Do NOT drive into Chicago with an RV of any type. Camp outside the city, preferably at a CG near commuter rail service, and use the train to go in. At this pace of the trip, your best bet is to pick what you are most interested in - nature? history? national parks? cities? scenery? And plan your stops accordingly. Good luck!
Almost forgot - remember to take your passports!!
I cannot even imagine trying to park an RV in the city, much less drive it in there. Closest CG to Schaumburg I think might be the Chicago/NW KOA out in Union. Do NOT drive in the city with anything more than the toad.