Consider asking over here: https://www.forestriverforums.com/forums/f232/
If you do not have an account there, sign up using the same email address and password as you use here. Both forums are owned by the same company so it'll find your account here. Complete the profile and you'll be good to go.
The driving experience on the Ford F53 chassis is dependent on a lot of things such as the age, the wheelbase, the GVWR, etc. Shorter wheelbases and lighter vehicles seem to have more problems with drivability. The F53 also has a solid front axle, not IFS. The first step is always a front end alignment by an experienced mechanic, experienced in motorhomes. I used the local Ford Commercial Truck Dealer and it cost me $135.
Then you need to figure out what problem you're trying to fix. A steering stabilizer, a device that uses giant springs to help keep the wheels pointed straight ahead, helps a lot of people.
Sway can be fixed with new sway bars.
"Tail wagging the dog", where a passing truck pushes you around, usually can be fixed or at least improved with a rear trac bar. The trac bar bolts the box to the rear axle, otherwise it kind of floats around. When the bow wave of the approaching truck hits the rear of the motorhome it pushes the box over and that pushes the motorhome. Then the truck passes and the negative pressure from its wake sucks the box back over and then it kind of re-centers itself. The front trac bar is already present. Yeah, we have wake turbulence problems.
A lot of the jarring and banging can be fixed by dumping the crummy Ford shocks.
You could put about $3,000 or $4,000 into fixing the ride and driving. I'm not quite there yet but I did add new Koni FSD shocks at 800 miles, a steering stabilizer right from the dealer, and a rear trac bar, also around 800 miles.
You're not really going to drive a gasser much more than three hours at a time anyway because you usually start looking for fuel at around 1/2 tank, which is about three hours for us.
The FR3 is really an entry-level motorhome and while a lot of people like them, they do seem to lack the little extras of the GT3 or GT5 series. For example, some of Forest River's entry units have a generator but no automatic transfer switch. You need to plug the shore power cable into the generator.
If your cousin is near you, your experience in helping them understand what they are lacking could help keep them from making a decision they might later regret. I think the FR3 has 19.5" tires which can negatively impact the ride.