I try and get my trailer into the dealer for a bubble test every year where they pressurize the trailer with a fan and spray a soapy solution over all the seals and joints. Sure enough, every year there's some spots that need attention, even on the roof.
One year the sealant on all four corners of the roof needed redone as well as the entire skylight seal. I usually pay the shop to seal everything up they find, which gets spendy, but it's a lot cheaper than having to perform a major structural repair or scrap the trailer entirely because of dry rot.
Josh
'16 Ford F350
'11 Komfort Resort 240RK
Portland, OR