The "service" truck the OP is talking about is that, a "service" Truck. I drive one that is my dd also. I have on board inverters that I use for numerous things like charging batteries, running a laptop, or just general use. Some trucks have PTO's that run on board air compressors or hydraulic pumps and such. There are countless reasons a service truck is left running and the OP stated that the back doors were open. The operator of that truck could have been in the back working. It may have been his mobile office.
This is what is wrong with this board. People always assuming.