I bought a new to me TT that is in really great shape and had very few areas of de-lamination. I noticed that the dealer the folks I bought it from who they got the unit from a year ago, went over it ...
I think the folks that have delamination and are still using the rig have had it in for a repair estimate and found out its a prohibitive cost. To me delamination of any kind is a deal breaker and I would not want to purchase one with the condition. Since you bought it and were aware of it I would do as much as you can to ensure no more leaks occur and just enjoy it as is. Unless its a tiny area delamination is not a simple or cheap fix unless you happen to be a diy fibreglass guy.