We have a 35' GMC coach. We decided in the shorter version so we can go anywhere, and we love the state parks. Buy just wait to hear what we ARE going to shove into this little space...
My wife and I, 3 boys AND 3 girls AND a standard poodle! Yup!
One big advantage though is, no wheel wells, rear engine, and a very large space above the drivers area. Rear queen bed room set sideways, side isle, 6x6 bunk room next for the boys, bath with shower, kitchen, two couches with a removable table in the middle and then a large bunk over the drivers area for the girls. The couches can be laid out to make another bed are if need be. Washer and dryer is going in the lower storage bays. I've laid it all out on the blank floor inside with tape along with the thickness of the walls and got it all to work =-O
So.... My point is a 40 ft should be plenty of room! You can do it!