Yep, you need to disclose what you know. When I bought my latest trailer (private party) I was assured the roof had always been maintained and sealed on a regular basis, also that the wheel bearings had recently been serviced.
I wish he had been truthful. I found some really weak spots in the roof as well as a tear over one of the rotted spots,and the bearings were not done.
I will probably sell the trailer next year as age is catching up to us and I will disclose the damage, as stated elsewhere I like to sleep at night and its the right thing to do even tho I know I'll take a financial hit.
treat others as you would like to be treated.