I installed four slide toppers on our 2010 Mountaineer. It would be nice if Montana incorporated slide topper attachments into the gutter, but, unfortunately, they don't. When you get your toppers, they will include the upper rail. I installed mine directly under the gutter in a letter "P" configuration. That put the top of the topper as high as possible (within specs.) to allow for proper water drainage. The only thing I had to do was temporarily remove the end spout (one screw) to allow sliding the topper into the rail.
The upper rail nor the topper attachments to the slide need a stud. The screws just screw into the fiberglass and the wood behind it. You will see that the many rail screws that come with the topper are only abut 1/2" long. It's not a bad job, just be careful to follow directions. I installed mine by myself, if you have help it will be even easier (depending on who your help is
) Good Luck