Delivery trucks would be interesting, they would be really good for groups for transporting supplies and would be equally good for a single player especially if they let you modify the trailer enough to turn it into a living space. There could be a couple ways you could take modifications, first you could go for a prue defensive purpose and armor the hell out of the thing this would be good for transporting supplies and providing defense for a building or door. You could also partially armor it and cut out some side areas of the trailer to allow people to fire from, this would allow you to turn the thing into a large armored troop transport. You could also use the trucks as zombie carriers like some people have wanted to do, you would just need a ramp and some come with retractable ones anyway and some way to open the trailer door to release the zombies.
Finally like I mentioned above you could turn it into a mobile hq, you could armor the outside and add a lot of improvements to the trailer like a roof vent, a portable generator for power, a bed, and a wood stove or electric heater and some crates. Since most have ramps as well you could park a small vehicle inside the trailer for later use like a bicycle, motorcycle or ATV, a UTV might even fit as well but you might need a bigger ramp.