In addition to the AC, I wanted to add a heat source for when we are camping in the early spring/late fall (or up in the mountains).
When it comes to bang for the buck, the knock off diesel heaters are hard to pass up. They will output close to 20k BTU on high, and sip fuel on low.
My solution was to build mine inside of a Stanley tool box, as well as add its own power and fuel supply to make a fully stand alone unit. The battery is an AGM for a Mazda Miata (32ah), the fuel cell is a 1 gallon and 4oz gas can, and the installation pieces were included with the heater. Now, I have a fully portable and self contained heat source.
When running in 85F weather for 30 minutes on high, the internal temps stabilized around 100-115F after insulating the discharge tube. This will keep the diesel and battery warm, without exceeding a safe level.
I will add a port on the truck to connect the heat output to, and the inside will be ducted in the cab and bed area.
Run time on the battery is roughly 10 hours, and the fuel will last approximately 24 hours. I added a battery charger connector to the outside of the box, so I can plug in to the truck (or battery tender) and keep the battery charged.