Sometimes you just have to step back and let loved ones find their own way despite already knowing they are making an expensive mistake. This may be one of those situations- only you know.
Having said that, here are my thoughts:
1. Since your FIL is disabled, can he really handle hitching and unhitching? Have the dealer hook up and unhook for him that 5th wheel using their service truck. Then after that lesson, have the FIL do it himself. That should give him a good idea of the work involved.
2. Actually go with your FIL to the campgrounds he plans to camp in. Take a tape measure. Or research on line the campgrounds he wants to use and actually call the offices.
3. No manufacturer deliberately builds a bad product. However, some manufacturers are able to produce a better product than others. Use the Ford, Chevrolet, Dodge analogy with their fixation on a Keystone.
4. Have them look at 1 and 2 year old used prices on the same or similar model they are considering new. Look at the depreciation. Is the loss of that much money worth buying new?
5. Suggest buying an extended warranty to compensate for the manufacturer's new warranty.
6. Reinforce the concept that any commissioned salesperson, regardless of how sincere they appear, are representing themselves in the sales transaction, and their primary concern is for the commissions they will earn for themselves.
