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now is a good time to change over to synthetic oil. change the oil and drive it for 20 miles and park it. have the fuel tank full. diesel fuel dosent go bad. spray the batt terminals with batt protectnt. a battery tender would help if it is possible to have one pluged in. if not, your batts will probably be junk when you get back. if there is sunight , a solar charger would do the trick. but only a small one.
If this was my situation, I would sell the truck. Get out of the payments and invest your funds for a purchase when you come home. You will have 3 years of depreciation, insurance, taxes with out the joy of using your truck.
The Corps does allow people to visit home ever once in a while so he may want to hang on to a vehicle.
If you've got a mustang and the truck that are just going to sit there though it might be a good idea to consider selling the truck. When I first left for Pendleton I left my truck sitting at my parents house but there were a few less things to go wrong with that 64 model compared to your truck. I'd be scared of having to chase gremlins unless you have it driven every couple of weeks.