Seat belts that are attached to the seat frame and are lap-belts only are about as much use as certain appendages on a boar hog! All front seats should have 3-point lap & diagonal belts.
Unfortunately the loads involved in a sudden deceleration accident can seriously damage structure if a diagonal belt is attached to something that wasn't designed to take the load.
Also, I don't think that Class A front ends meet car-applicable crash-worthiness rules, so, if the the front end is the impact area, the belts wouldn't be much help.