If you have Good Sam, have you used the "Trip Planner" on the Web site? Despite its 25-stop limit per trip, I have found it very useful to plan trips or segments of trips. It has built-in databases for historical sites, campgrounds, Camping World stores, etc., within 1 to 60 miles (you choose the distance) of your selected route. This may help your find some less well-known historical sites and less expensive campgrounds than you might find just doing searches by town, region, or even state. And you can constantly update it or replace it anywhere you have WiFi.
We learned one solution to washing clothes from a couple of "winter Texans" we met several years ago. The camping limit for the Padre Island National Seashore campground was 14 days, so the couple carried 15 days worth of clothes. When their 14-day limit expired, they packed up the trailer and went to a private RV park with a laundry in Port Aransas. They did laundry, shopped for 2 weeks worth of groceries, got haircuts or did other personal shopping, maybe went to a movie, ate a nice restaurant meal, then headed back to the inexpensive NP campground for another 14 days.
The same thing could be done even if you are not going back to the same Federal or state campground each time. After all, teenagers will likely need to have a "town break" at least every couple of weeks (at a minimum). And it's good for mom to get away from cooking a meal for 4 once in while too, even if it is just at McDonald's!
Having a separate bedroom for the teens in the garage is a very good idea, but will they ever need a break from each other? If they have separate bedrooms at home, you might want to consider keeping that loft for "sleeping quarters" for one of them. Not disparaging your teens, but people in tight places do tend to get on each other's nerves after a long time together. I remember how much "space" my wife and I began wishing for by the end of our 3-month, 24/7 Alaska-western Canada trip in our 21-foot travel trailer. I also remember that telling our 3 boys "to go to your room" ended a lot of potential arguments. But that was a long time ago--pre- cellphone, WiFi, Tivo, XBox--so maybe things have changed.
I installed an extra CO detector near our bed because the factory one was at the other end of the trailer. I also do my own trailer maintenance. You likely already have a list of essential tools for doing your own maintenance on the trailer. But, on the Alaska trip, I took (and used) a full set of mechanics tools--ratchet, sockets, open and box-end wrenches, etc., a small hammer, all kinds of tape (duct, Teflon, electrical, scotch, etc.) and Gorilla glue, 12v tire inflator/compressor, tire plug kit, grease gun and extra tube of lithium axle grease.
Of course, you may not be in as many remote areas as we were, so may always be able to get to a Walmart or Home Depot to pick up anything you need. But, if your trip takes you to places like the national parks and monuments in northwestern New Mexico, southern Utah, or northern Montana, you probably should assume that it will be "Alaska-like" as far as convenient distances to a store and pack accordingly.
And, just FYI on historical sites, there is arguably no place in the U.S. that would give a teen a better sense of ancient American "pre-history" than Chaco Culture National Historical Park/World Heritage Site. It is one of the least visited Anasazi sites in the West and one of the very best. If you decide to go, plan on at least 3 days in the campground there to take advantage of multiple ranger-led tours, hikes to some of the outlying ruins, and a night sky program. You won't be disappointed!