I have never installed seat belts in an RV but..... I have in a car.
Just make sure you have a good solid anchor.
One option might be to install new captain's chairs.. they often have sea belts attached to the seat itself.. now you won't have a sholder belt (I've never installed one of those) but the lap belt is way better than OH SH!!!!!!!
As to steering.. AN alignment is always in order, With very few exceptions every vehicle on the road could stand to visit an alignment shop.. A good alignment INCLUDES a complete inspection of suspension and steering components.
THEN adjust theball race (The screw in the nut someone else mentioned)
Something else that might help (3 well really 4 things actually)
Sway: Side to side rocking of the motor home as you go down the road (Think of a bunch people singing some song or another holding candles weaving side toside in unision)
Sway bars counter that.. Many older rigs have good sway bars but they may have aged...
"Wagging" think of a dog's tail as he runs toward his master) Trac Bars, the standard wisdom is to put one on the rear first,, Then if needd add the front trac bar. I put on only the front bar.
Steering Stablizer.. This is a device that tries to FORCE the rig to go down the "Straight and narrow) Tries to keep the front wheels centered forward) NOTE that proper alignment will help with this as well. I have a Blue-Ox True center. This unit can exert something like 275 pounds of lateral pressure on the tie rod to center it.. and if, due to road,load or wind conditions you need to adjust the "Center point" the job can be done in less than 30 seconds WITHOUT REMOVING YOUR SEAT BELT.. In fact.. It must be done in less than 30 seconds (There is a timer on the adjuster that is set to 30 seconds) The older True-Centers used to burn out solenoids.... They told me they were trying to figure out a way to fix that, I suggested the timer. Guess they liked my suggestion.. Wish I'd contracted for it
Note there are other brands..At least one of which can be easily adjusted, though I doubt it's that easy. There have been some problems reported with the True Center. (I reported one of them, Blue-Ox replaced it, only charge was one way shipping and labor...I did the labor)