I don't think you can go wrong with the spray in liner. My 2500HD came with Line-X and I haven't had the camper off the truck in a while to look for an problems.
Underseat storage: I thought about those, but instead I removed the plastic facia pieces on the outboard sides of the rear seats. That way I have access to most of the stuff I put under the rear seat without having to flip the seat up. Things like a fire extinquisher and first aid kit I can grab just by opening the rear door of my crew cab.
If I didn't have a stock overhead console, I would get a shelf-it for extra storage. I wish they still made visor-shelf-it, as I really like it on my Dodge.
Poweramp steps: Whatever it takes to keep your wife happy! The way the cab sites on the frame rails, I've thought about some permanent non-moving steps. But haven't found anything that I really like. I may eventually fabricate my own as I've recently started welding.
Airbags: I found that the camper dropped the rear about an inch, which was perfect to level the truck. I couldn't get my Dodge 1500 into the garage with the camper, and there is no way I can get the Chevy in the garage. I haven't measured my garage door height, but I'm sure it's less than 8 feet. I put a 4 inch lift on the front and 3 inch on the rear and as I recall, the height of the top of me FWC Hawk is 7 feet, 6 inches. Add 3-4 inches for the dome of my fantastic fan.
If you have a diesel, you might want to look at aftermarket fuel tanks. I have a 52 gallon fuel tank, which is almost twice as much as the 26 gallon stock tank.
I'm happy to carry the extra fuel weight for the extended range it gives me. Maybe someone makes bigger fuel tanks for gasoline trucks, but all the ones I found specified for diesel only.