In reply to no_more_scotch_eggs:
I did a 10 week trip few years ago mainly in Utah and Nevada, with extra trips up to the Tetons, Yellowstone, Yosemite and Highway 1 from San Francisco to Los Angeles. My favourite places were the desert state and national parks - Arches, Canyonlands, Dead Horse Point, Zion and Bryce, and individual sites such as the Wave and Antelope Canyon. And it would be rude not to mention the Grand Canyon, which despite being quite touristy, is definitely worth the effort.
I'd second the recommendation for the Moon guides. When you're researching the places you want to go, watch out for the permit requirements. Some places only issue a limited number of permits per day. We got very lucky with permits for the Wave and the Grand Canyon, managing to get them the day before we wanted to do the trips, but a lot of people will book a long way in advance.
I was there in April/May/June. In April, it was warm enough to be hiking in t-shirt and shorts, but I did get snow at higher altitudes on my way to Bryce Canyon. By late-May, it was very hot - days over 40C were not uncommon. I do recall one day driving for half an hour from my campsite to a shop just to feel the benefit of air-conditioning. By July/August, I suspect it would be hard to stir yourselves into action.
I loved the desert landscapes. Completely different to anything I'd encountered before (despite a couple of trips to Morocco), and I'm contemplating a return visit in a couple of years, if I can overcome my aversion to US immigration interrogations on arrival. Try not to bite off more than you can sensibly chew - there's so much to see, and you'll ave to make choices between cramming in as much as possible and getting exhausted, or taking a slightly more leisurely approach and enjoying things a bit more.