Do you have the relays in the required positions?
According to the fuse information section near the end of the 2015 Explorer manual (the year I happen to have on file), the relevant fuse/relay panel positions includes #12, "Trailer tow battery charge relay"... in addition to the 30 amp fuses in #24 (for "Trailer tow battery charge") and #3 (for "Trailer brake control module"). There are also relays and fuses for the trailer lights, but you must have those because the lights work.
If you don't have the relays, perhaps you have a wiring harness which is missing some connections, including the pin for that fuse. Does the vehicle have the optional (but included in some trim and engine combinations) "Class III Trailer Tow Package