Just like the old days- for the seat belt to lock and pull like holy hannah. You can get seat belts just as tight as the latch system. When my daughter was born (she's 10 now), we used the center seat in our Durango because it was safer than being beside of a door. The problem was, there wasn't a latch system for that seat. We could rock the SUV by grabbing the car seat and shaking it. There wasn't any movement at the base of the seat.
If you go online, you can actually find "professional car seat installers". No, I don't make that up (yes, I know back in the 60s kids road on the back deck of their daddy's station wagon). We paid for her to teach us to install car seats properly. We've actually had friends bring their seats and cars to us to help install them. You could likely find someone local that does it and have them install it.
Harder to find are car seat inspections done by the fire department. Once you get that bad boy installed, contact your local FD and see if it is something that they offer. I'm sure the fire fighters would love a few minutes in the RV to check it out (the camper and the seat