Two weeks is just fine if that's all the time you have. We've done it often with the kids in our early years.
You stated you'd like to do the Southwest. I would suggest concentrating on the southern Utah national parks. Driving distances once you get there are not far and each park is so different. There's Zion, Bryce, Escalante, Capitol Reef, Arches and Canyonlands (which you can do from one location).
With your 29' TT you'd easily be able to fit in all of them and that would be your best experience - staying in the parks themselves, and would save fuel. The teens (& you, too) would absolute love these parks. Definitely take your own RV rather than thinking of renting. You're used to it and in the long run, it will be cheaper.
Have fun planning!