Family road trip for 6?

This summer my family (grandpa, grandma, mom, brothers (age 26 & 13) and I would like to do a summer roadtrip sometime hopefully in July. we always do national parks and we take the RV so we dont pay for hotels or resturaunts. We cant seem to find a nature/ attraction filled place to go. We live in Las Vegas, NV and would LOVE some ideas. (We've done the Red Woods, Yellowstone, Yosimite, and Lake Tahoe already) Thanks so much!