We have a 36' gas motorhome and when my daughter,hubby and 3 kids come to visit, it is crowded inside. When the weather is bad you will all have to be inside. We can seat 10 putting 4 people in the dinette booth - not very comfortable though. K Most motorhomes are comfortable with 2 people, maybe 4 for awhile.
If this is going to be full-time, I think the adult children should stay where they are - get a place of their own - go to college, get jobs and get started with their own adult life.
We purchased an older quality motorhome when it was 10 years old. Have been full-time for 8 years in it - no regrets. You can find quality rvs in your price range if you take your time to look.