I am the second owner of my Bigfoot TC. I bought it in 2004. With the exception of last year, it has never been under cover. It did sit under an Oak tree at least seven years that I know of just before I bought it. It could have been longer. I had a leak in the caulking in two windows several years ago. I re-caulked all windows as well as the rest of the TC. I found no dry rot. I believe the molded fiberglass greatly reduces the potential for leaks. I have owned stick TCs.
As for my '04 Dodge (104000 miles), if it does as well as my '96, I will be tickled. It has about 250000 miles, and the only repairs have been radiator, timing gear seal and transmission.