"catkins"....You're going to get answers all over the place and here's another one. First, the engine heater isn't necessarily for single digit weather. It helps in lower temps to keep the coolant and oil warm so that your engine doesn't start with thick oil and not lubricate as fast as it should.
This was discussed before and I think there was an actual recommended temp by Cummins to turn the heater on. I believe it's somewhere around 25 degrees. You can still use your engine block heater when boon docking. Obviously, it only needs to be used on the day your leaving. Start your generator and plug in the block heater when you get up and while having breakfast and getting ready to go. About two hours should help. If I have hook ups, I plug it in the night before.
I know Az can get cold, but as one poster stated, you probably won't see sustained cold unless you're in an area like Flagstaff. If you're really concerned, add the anti-gell and don't worry about it. Keep in mind, once your engine is running, it will heat the fuel on it's own. Diesels always pump more fuel to the engine than they use and then return the warmed excess fuel back to the tank.
Good luck with your new diesel. You were smart to come here and ask. I'm on the west coast and luckily don't have to deal with the fuel or winterizing issues.