It looks like a small box with two terminals on it. (see photos)
On a travel trailer it is under the frame and on a 5th wheel it is located under the pin box (in mine). There is another located in line before that close to the battery, but because your slides work, that one is good.
Look for corrosion at the terminal or a popped reset button located on the side.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW