Hello,
we have a 1970 LHD westy & we had the same quandary. Our eldest (7) sits in the front with John (hubby/driver) with a booster seat using a recoiling seat belt, as what was fitted as standard wouldn't reach to the seat belt clasp with car seats. Our son used to have seat stage 2 car seat that needed a 3 point seat belt & all worked well.
Our youngest (5) sits on the R&R bed facing in the direction we're driving, on the left hand side, in a stage 2 car seat, which requires a 3 point seat belt, as she is not tall enough for just a booster seat. I sit in the back with our youngest.
We looked at lots of different options when we got our bus but it come down to safety. We couldn't mount another 3 point seat belt in the back as we would have had to mount the seat belt on the wardrobe or the other non workable option was to find another mounting point underneath the R&R bed and feed the belt up behind the seat & over which wouldn't have worked as no belt would be long enough.
The buddy seat option is not an option as you're not supposed to drive with two people sitting back to back car seat or no car seat.
Not sure if i've helped you or not??
Biddy :?