We just came back from our San Francisco trip where we stopped at National Parks on the way out. We came home well under our budget which made some things very clear:
We bought an annual Park Pass. It was $80 and we only ended up going to 3 parks. In the long run, it only saved us $10 instead of $40, but it sure was convenient.
My wife did an awesome job shopping around for hotels and prices. Every hotel was reasonable, and they all had breakfast. That is a big deal with a family of 5. That saved $40/day, easily. The best hotel decision was on SF. We didn't stay in SF. We stayed in Oakland. We had to drive across the Bay Bridge to get to and from our hotel, but at over $100/day savings, it was totally worth it. That hotel was super nice and very well priced for the area.
Unless you're hiking a NP, one day there will probably cut it. This is personal opinion, but I can only look at so many trees, valleys and rock formations before I'm bored. One day each, with only a few hours at the Grand Canyon was perfect. That would reduce your need to leave and then go back.
We also passed a load of KOA Campgrounds. I have heard their prices can be good and bad, but calling around will help. Do all your footwork with Google.
As for being gone for 2 weeks, that's a lot. We cut our last park stop out because we wanted to get home and were tired of the road. On my trips to EOM, I'll screw around on the way out, but on Sunday morning, I'm focused on getting home and haul ass to make that happen. Those are usually only 7 day trips, so another 7 would be hard. Last year, I went with my brother and his GF, and we were gone for almost two weeks.
You can break that up with visits to other people. You could come by here, stay the night, have a good dinner, and we could ride the next day on your way out. Or, you could plan the first week of the trip very well, and have ideas, but not make any reservations for the second week. Then if you want to go home, all you gotta do is point in that direction and throttle it. Vacation mode and "get home" mode are two very different things for me, so I'll shoot for larger cities and grab a budget hotel. Free breakfast is still a requirement.
For two different trips, my wife flew out to meet me. Once at WFO and once at Wheaton's First Camping Trip. That made being gone easier, but I start to miss my kids also...and my own bed. Either way, the flights were easy and we had a ton of fun. Haha...