I built one out in the last 2 years. 2015 Sprinter. Love it. Kept it very simple. Sound proof, insulated, 1/4 plywood, 1/8 foam and material over that. One bed is the seat that pulls out and on top of that is a fold down bed. this takes up less room by folding up the bed. Use standard kitchen cabinets, but cut 4" off of them. Have a sink with 12 Gal. of water and gray water all stored under the sprinter.