Now, I'm going to assume you only want to convert the front seats, so I'll go from there.
Go to your local junkyard and try and track down a late 80s, early 90s Dodge Van (They still had the 70s body style up till around 92). You'll need torx bits for an impact wrench or socket wrench (You may also want ot bring your son) remove the plastic covers from over the large torx bolts for the seat belts and remove all three (Two for the main belt and one for the buckle).
Bring an cutting torch, you'll want to photo graph and then cut out the section of the door frame where the upper shoulder harness attaches. This piece will later need to be fitted and welded onto the cab frame of your motorhome.
Basically from there, go home, remove the bolts holding down the lap belts and buckles and reuse the original bottom to to secure the new belt and buckle in the lower locations.
Cut your existing upper door frame structures so you cna weld in the upper seat belt mounts, grind and paint to finish.
'92 Dodge W250 "Dually" Power Wagon
'74 KIT 1106 Kamper Slide-in Truck Camper
'06 Heartland Bighorn 3400RL Fifth wheel
Follow along with me in the The Journey of