You will need to distill all your camping experience into an ordered list of priorities.
My priorities when I got my Itasca (my first RV) were to have a place to sit and work/eat, have a separate comfortable place to relax/play my banjo, a bathroom I could actually use, and to have a permanent bed all in as small a package as possible. That's exactly what I found in my Itasca and I am very happy with it. The one criteria I didn't press was counter space. I really wish I had more but not at the cost of a larger RV. Also remember to check out the cab space. I wanted a gas engine and worked to get a Chevy chassis rather than Ford to have more leg room.
The bonus I got without really understanding it's full value is a large external storage area. Now that I have it I don't think I could get along without it.