You use Dexron, Type II, III, IV, or V. DO NOT use Dexron VI! Dexron III would probably be best, since it is closest to the OEM Type II. I've found Type III online.
Fill the reservoir to between 1" and 1/2" from the top with the trailer top DOWN. This means you need to fashion a kind of "dipstick" you can slip in there to see the level. I used a drinking straw that I bent over to fit in.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew