JCD - If your truck CAN supply the 120V AC power, then I don't see any problem powering the fridge with it. I think my new truck could do this as well, with the onboard 120V outlets in the bed. I just don't see the need with my solar power setup.
I don't know that 12V DC is more efficient than 120V AC. The MOST efficient way to power the fridge is with propane, from everything I've read, but this is not a good idea since having an open flame at a gas station is a recipe for disaster. I've always thought that 12V was the LEAST preferred way to power the fridge, but that is probably based on how quickly it will drain the battery.
Since you have the capability, I'd say go for it and see how it works. You'll be powering your trailer's converter too, so it will be charging the battery along with your truck's +12V DC output. I don't think this will cause any problems, because I think they can work together.
Sorry for being pedantic. I think folks need to know that old appliances that were designed for the 110V standard may not be usable now.
- Jack