This thread comes up all the time. Here's the posts from last year. http://rib.net/forum/showthread.php?t=32954
Getting access to the hull is half the battle. When it's on a trailer, you have limited access and can't really use any force or power tools as it's at a weird angle (rollers on the trailer get in the way, etc). Thinking aloud, it might be worth removing the engine, so you can get the boat on its side. The boat could rest on old tyres, covered with a tarpaulin. Just a thought?