Mike,
I think you have your trip well planned ahead of time - we share many of the same traits. I always reckon on the high side of things as well. As a matter of fact, I've figured on gas being $3.90 this May and this summer and have that factored in to Microsoft Streets & Trips with $100 a day for "normal" activities (CG fees, small extras). I haven't figured in the costs of paid tours (Grand Canyon, Sequoia, Yosemite) as I already know the cost of those and have a separate line item, so to speak, for that sort of thing. I always allow the first tank of gas that I get prior to leaving home to be a "free" one as I always gas up long before I leave. May not make a lot of sense, but that money doesn't come out of my trip money. We always stay in one place for a minimum of a month in order to take advantage of monthly fees, it makes a huge difference. A big difference that differentiates me from many is that I don't drive over 60 (in fact, many go slower) and that helps a little, although I've driven at sustained speeds of anywhere from 55 to 65 to determine my "sweet spot" and it is 60. It makes little difference if my generator is on full time of not (and it frequently is when I travel west), I get between 7.3 and 7.6 mpg.
It's nice to see someone besides me who plans ahead (I'm on the anal side!). In my case, although we are gone for at least four months every year, my planning for the following year begins within a month after my return home, including getting reservations for the following year. There are many decent software programs that will allow you to plan trips. My primary is Microsoft Streets and Trips and the secondary is the Trailer Life CG Directory (it also includes a trip planner which isn't too bad).
Good luck on your trip and I hope you have as much fun as it looks it will be. If you go over your budget, just remember that especially with young kids, stuff happens, so just roll with it.