Take off the grab handles on the rear corners and you will have three 6 mm bolts to mount a plate to. I took a $.89 electrical box cover 4.75" X 4.75", cut down on side about an inch to make it fit in the area. The cover has a preformed 3/4" hole in the center to knock out for a NMO monut.
In my first setup I Drilled 3 holes in the plate to line up with the bolts, used some 3/4 " teflon space to raise the place up enough to allow a 239 connection. Also bought some 6 mm X ~ 1.25 inch to replace the factory bolts.
In the second setup, I used a long antenna that needed a swivel mount to lower the antenna for parking garages. I picked up the Comet RS730 lip mount and clamped it to the plate. Yeah, It sounds funky, but has a better SWR than the first setup and is likely from having the antenna up higher and in the clear.
Yes it is somewhat directional, but it works very well for the garage challenged. AND you have 2 of these mounts just waiting for antennas.
My first setup is very similiar to this one although it is a CB ant.http://www.geocities.com/wknaylor/Av/Ant.htm
Also, here is a link of the more valuable info regarding communications equip in AVs.http://www.chevyavalanchefanclub.com/cafcna/index.php?board=162
The site has other items that could be used for antenna mounting like a luggage bar that runs across the bed covers from grab handle to grab handle and a light bar that bolts with the existing ones used for the cladding on top on the sails.