Just so you know, Dealers and the manufacturer are notoriously wrong. RVs get re-configured so many times, they really don't know what it weighs. Also, who knows what you typically haul in it when you are fully loaded.
Some of the reason you are getting the answers you are getting is because it is so well known that under inflation is such a huge cause of blowouts. It is a big deal, and those who have been around for a lot of years will concur with reports read on here. Personally if I were riding with you and had to choose between slightly under inflated tires, or towing twice your rate limit, I would choose towing twice your weight limit. Under inflation is that serious.
Yes, the F53 is known to ride stiff, although that has improved in more recent years. It would be interesting to know what your GVWR is, compared to your actual weight. As you mention your coach is pretty light, that may be part of the problem. There are better ways to address the problem, although not cheap. MorRyde is one option. Sulastic spring shackles is another option. Some have been able to just change shocks. A little searching here should give you what others have done to the late model F53.