Welcome to the group. I have an 18LT, and while I haven't thought about the walls, I have contemplated adding insulation under the floor. I just need a CHEAP source for some of that plastic cardboard/sign material that I can use to hold up the insulation, and make it watertight. The only way on mine to add wall insulation, would be to remove the GLUED on wall panels, and glue on wall board, then panel over, or some such as this. Since I have an electric bed, would be difficult, removing, shortening, blah, blah, blah....
Good luck, keep us posted if you try it. Remember, cold air goes down, so the floor is why is why I was thinking I would get the most gain there.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT