[Bob Landry]

As a Dometic servicing dealer in the marine industry, I deal with a lot of manufacturer defects, warranty claims, warranty rejections, and unhappy customers.
Warranties are specifically written, usually to say that the product is covered for one year or two years, whatever, against equipment failure and manufacturers defects. After the period specified in writing is past, the warranty goes away. it's like that in every industry I know of. I have gone to bat for customers in similar situations as this one and in some cases where the failure happened or was noticed close to the expiration date of the warranty period, I have been able to get my customer the assistance he needed, other cases, I have not.
Two years beyond the warranty expiration is pretty significant, and while I can sympathize with the OP, for any company to handle it any other way would leave them with an open-ended warranty for the life of the product. No one can stay in business for very long doing that. What if the OP had owned the trailer for 10 years before noticing the defect instead of three? If a shop looked at it and declared it a manufacturers issue, should the manufacturer still be responsible for a defect that had gone 10 years without being noticed?
Concerning the Keystone warranty, it specifies that seals, caulking, etc should be responsible for inspecting the RV every six months. If it can be proven that was done, then the OP may have some leverage, though unlikely after three years. If it had been done on a timely basis, the question would be, why is it just now being noticed? This doesn't sound like the type of failure that would occur in a matter of a few weeks.