The first RV rack I made mounted a pair of Sports (about 10 feet) diagonal across the back, stacked 2 deep. It was too difficult for me to load, so I made another rack. That diagonal rack did fit the kayaks pretty much behind the coach, and I even had a 13 foot Classic on it at one time (which was actually too tall for my tastes).
Figuring 8 feet wide and 8 feet tall (body), you've got 11.3 feet on the diagonal. The problem is..... taillights and license plate, which means you can only use the "middle" 6 foot wide section of the coach. Using the 6 foot width and 8 feet of height, you can get 10 feet of the kayak behind the body (on a smaller diagonal), with an extra 1-1.5 feet sticking up. This is about the height of your roof A/C, and what I've been using for years on my current rack arrangement.
Go to
http://home.earthlink.net/~leinfam/index.html and look at my current rack setup (some pics are from my older, smaller motorhome). I now use a swinging gantry arm, with electric winch, to load each kayak (easier and also safer when loading in high wind).
My thought would be a lower cup (for the bow or stern), mounted from your trailer hitch (offset mount), and an upper carrier (padded U-loop) mounted to your ladder, if you have one. Otherwise, you'll need to build from your hitch (or bumper, like I did). You could use a simple block and tackle to swing the kayak into the upper mount, if you made the lower cup mount with a pivot (unless it's easy enough to swing up by hand).
If you need to put the spare underneath, like I did on the last 2 coaches, use a "chain hoist spare tire carrier" from old Toyota pickups in the junkyards. More than strong enough for the big spares, and no crawling underneath to take out or put away the spare. They bolt onto a steel plate (you'll need a 12mm wrench to remove it)...I made a simple frame clamp on a crossmember tube, and bolted it tight after the spare was properly positioned. Wear gloves while pulling one in the pick-a-part, and if the bolts are too rusted, just pull tighter until they snap off. You can easily weld the hoist body right to the new carrier support......no worry about bolts coming loose, that way!!
Hope this helps.
Regards, JimL