karebear - I presume you mean the the inside walls are a little over an inch thick. And...you think the back wall is about 2 inches thick. Your exterior walls are actually constructed with square aluminum tubing welded at joints. If you can determine the location of one of these aluminum tubes a self tapping stainless screw would hold your hook very well. For fairly light items, you might try the self drilling plastic drywall screw-in anchors.
richardkaren - Congratulations on your purchase. I hope you will be able to enjoy your unit as all of us who have purchased the so called "B+" size. Which really is a narrow Class C and usually does not have a bed over the cab. The 3,000 lb boat likely will not be a problem and will give you that extra measure of enjoyment at your destinations.