Holly, there are other options to using foam board for insulation. Do a search on "Reflectix". It's used in Scamp
(and undoubtedly others) as an insulated backing for the fuzzy carpet-like material lovingly known as 'rat fur' in Scamps. It's a metallicized plastic bubble wrap that has excellent insulating qualities, is fairly easy to work with, and will bend around compound curves. It may be a little more expensive than the foam board, but it will come in a roll, and it'll certainly be easier to use.
The problems you have with leaks
in your Bigfoot
just come with older travel trailers, period. I have had a number of them over the years. The difference with the fiberglass trailers is that the little leaks don't cause the structure to rot and deteriorate. That said, every through-hull is a potential leak spot, and I've even had some on my '02 Scamp
already where the <gasp... this is a four-letter word around fiberglass...> silicone sealant used at the factory on some screws didn't adhere well. DON'T use silicone for ANYTHING on a fiberglass trailer in your quest for sealing leaks!
It doesn't sound like you have anything significantly worse than most of us have seen at one time or another in a trailer the age of yours, regardless of brand.