Hi,
If the dinette is not on a slide, it should be a bit easier to accomplish. Our rig came with plenty (12) of seatbelts up front (driver, passenger, 3 on each of the two sofas & 4 on the dinette) but I did install some on the bed to secure the kids when we were travelling at night. I used an online store, seatbeltpros.com or something like that. That was over 15 years ago. They had many options: lengths, attachments, colors, etc.
Ideally, you should bolt them through the floor if you can access the underneath of the dinette floor. Keep in mind that seatbelts in an RV should be considered as aids to keep the butts in the seats for emergency maneuvers, but are probably of limited help in a major collision where the structure of the RV is being ripped apart.
Due to liability issues, it is doubtful you'll find any company willing to install them aftermarket.
~Rick
