2004 Chevy Express 3500 15 Passenger - Decided I wanted a Chevy Express van, primarily for the better fuel economy but also for the design. I elected for the extended too since the two extra feet could come in handy.
The walls were filled with 1" hard foam and held in place with expansion foam. If I were to do this again I would elect to use spray foam instead, the time savings alone would be worth it. The paneling is 1/4" plywood with a good face.
CheapMessy and time consuming
The floors have 3/4" hard foam insulation and a 1/2" plywood sub-floor. The sub-floor was screwed into the chassis with 1.5" stainless steel lag screws, which were counter sunk into the wood. Then a layer of wood filler and one foot square vinyl tiles.