Multiple possibilities, Juno by Outfitter, or an Apex with pull out bunk. Hallmark 8 or 8.5 footer or a 9.5' Cuchara with an aux bunk over the dinette. Coyote RV will build a custom however you want. Northstar makes a model with a tent camper like bunk that folds out on the side. For a family of four, you will probably be cramped on a short box truck.....something on an 8 foot box would be roomier. If hard core off road is a priority, an 8 to 8 1/2 footer on a 6 3/4 foot box truck would probably work in a pinch.
A ship in a harbor is safe, but that's not what ships are built for.....