IcePyro-
I recommend against the idea of installing seatbelts for the forward dinette seat.
Seatbelts protect occupants from "flying around" in the event of an accident. I don't know for sure (I'm not a vehicle engineer), but I suspect the most common accident case for which the seatbelts are designed is a sudden stop from high speed. In that case, the passenger mass moves forward until restrained (or not).
In the case of front-facing seats, this means the seatbelt tightens. In rear-facing seats, the seatbelt loosens.
The difference is crucial. In front-facing seats, the seatbelts take the load. In rear-facing seats, the seat takes the load.
Here's the rub: The rear and front dinette seat structures can be flimsy and lightly attached to the slide floor and exterior wall. They were on my coach.
See the attached picture. It shows the steel truss Monaco built under the rear dinette seat to transfer the seatbelt loads to the floor and rear slide wall. Monaco could have chosen to bolt seatbelts through the slide floor, but didn't. The structure also ensures the belt load goes (roughly) across the hips, instead of mostly downward, which would be the case if the belts were bolted through the floor. This also reduces the chance of seat structure collapse.
In my opinion, during a high-speed stop passengers in the front dinette seat, belted or not, would break the seat back and bottom and essentially lose any protection the belts might provide. That's why I recommend against adding seatbelts in that position.
That issue aside, should you look under the slide you may find the through-bolts for existing seatbelts use an unfamiliar bolt design. It's called an "elevator bolt" (as in, a grain elevator). You can buy them from several sources; I prefer Bolt Depot (link to their elevator bolts
here).