I'm thinking the first thing I would check is, with the slides fully in and all the jacks retracted, like when hitched up and ready to roll, is the fluid level in the reservoir. No idea what model or year your rig is, but on ours, the fluid is supposed to be about an inch below the filler cap on the hydraulic fluid tank. If it is low and you do add transmission fluid, you may need to cycle the slides and jacks as you may have gotten air in the system. Do not worry about mixing fluids, as the rigs usually come with hydraulic fluid that has no dye in it, and ATF has dye, along with some transmission specific additives, but it makes no difference with these systems. Below is a link to the Lippert Lipsheet that gives you options on which fluid to use.
https://www.lci1.com/images/support/lipsheet/0291.pdf