If its registered as a passenger vehicle, all seats need proper seatbelts and anchorages.
If your van is registered as a motorhome, you need complying seats and seatbelts for as many berths in the van.
Presuming you only had two front seats, and 2 berths, any seating behind the driver does not need to be certified.
If it is a 4 berth van and you have only 2 front seats, the extra 2 seats in your rock'n'roll setup would need 3 point belts and anchorages certified.
I have had a passenger van (8 seater Mazda Bongo ) converted to a 2 berth motorhome, (with 2 legal seats), and more in the back, not requiring certification, its a five minute job at the testing station.
Toyota Dyna 100 coachbuilt A to Z Japanese built camper
2.4L diesel (2L motor)
Self contained (70L fresh, 35L waste)
80ah deep cycle battery