Bit of a specific one this. Does anyone know of any companies in the south west that can fit 3 point seatbelts to a motor home seat that currently only has lap belts?!
You'd be better asking on something like outandaboutlive.
It will likely only be possible on forward facing seats, and will depend on the layout and what is under the current seats and under the floor. A lot of metalwork needs to go in.
Often a new m/home is the answer.
You can buy aftermarket seats with 3-point belts but bolting them to/through the floor will involve spreader plates. I had this done to a Renault Trafic. In fact, I still have the seats in my garage.
"bolting them to/through the floor will involve spreader plates"
This relies on the floor being metal.and not ply/fibreglass like many coachbuilts