You're ahead of me build wise as my van is still on order but one strategy I plan to research more is to buy one of these harbor freight loading ramps, split it into three sections by removing the hinges, and then spacing each of the resulting 15" wide ladder frames with 1/2" birch ply over top to create a ~60" wide queen bed. They would likely need to get cut down a bit lengthwise and then supported with aluminum angle or channel ledgers fastened to the walls of the van.
1500 lb. Capacity 45 in. x 69 in. Tri-Fold Aluminum Loading Ramp
Amazing deals on this 1500Lb Aluminum Tri-Fold Loading Ramp at Harbor Freight. Quality tools & low prices.