DOT seats are only required if:
1) You are re-selling the van and need to claim FMVSS compliance for passenger vehicles
2) You are driving commercially and the van seats more than 9 passengers.
RV's do not require seating compliance. Most class A motorhome seats are nothing more than a Lazy-boy recliner lag-bolted to plywood, surrounded by shiny cardboard.
It's easy to install compliant seatbelts if they are separate from the seats. I personally would not install a seat with integrated seatbelts unless if were compliant, either aftermarket or a take-out from another vehicle.
2000 E450 dually V10 wagon