Pulling a Fifth Wheel problem, worried about my Dad...

If your Dad has towed with the same rig that long, he should be able to sense if something has changed about the rig over time. You should drive with him and let him describe the problem.

...however, don't rule out he may just be getting a little older. I have found towing can be very draining and as I get older it becomes more stressful and tiring.