Ah, ok. I'm sure it's very doable. At the Expo Rally in Hollister Hills SVRA a couple of years ago, I saw a setup where someone had a FrontRunner Roof rack with a tray mounted underneath that housed a big solar array (18x36" or 24x48"?) which was used for charging his battery system through a charge controller. The whole setup was pretty slick. Only thing I didn't like was it was pretty much a permanent system. OK if you only use your rig for wheeling. Most of us though work for a living driving other cars for better fuel mileage. LOL.
That said, Grasscat has a simpler and far cheaper setup to keep his fridge running in camp. He uses a 2nd smaller deep cycle battery mounted in back to run his freezer fridge after hours. During the day, he runs the fridge & recharges the smaller battery via the rear 12v socket. Simple and very cost effective. I think it's like this because it was designed by an engineer tasked with finding a solution to a problem -vs- finding a 'sell-able' solution to a problem.