Clear full is much better than clear empty. A full tank doesn't leave room for the moisture in the air to condense on the walls of the tank and lead to corrosion... that said, three months won't cause the problems a year or years will, especially in dry Arizona.
For a three month outdoor storage, I'd just change the oil (the chemical changes that happen to old oil and water from condensation can cause an acidic reaction), top the tank off, and disconnect the batteries. Diesel fuel won't go bad in 3 months so that's not a concern.
When you go to get the truck out of hibernation, I'd recommend pulling the pink wire off the IP and cranking it over for about 10 seconds three or four times to get oil flowing all through the block. Then hook the wire back up and start the engine. Let it idle for a minute or two and shift the transmission through the gears while sitting to let fluid pump through the tranny before starting off.
Later,
Joe