Here's the link
to the 2003 E-series trailer tow wiring diagram on the Ford Body Builders Layout Book page. If your coach is built on a 2002 chassis, here's the link
for that page. The information should be the same.
In the 2003 and earlier chassis, Ford provided only two connectors. The coach builder (Thor) used these for both trailer and coach lights (i.e., turn/stop, parking and backup). What that means is that in those year coaches you must assume that the trailer wiring does not go directly from the connectors to the trailer hitch plug, and modifying the trailer hitch wiring involves some rewiring on the coach side, as they have a common source.
If the 2004 and later chassis, Ford provided the same two connectors, plus a third connector dedicated to coach lighting functions.
If all you want to do is to add a charge line, you can run a 10 AWG wire from the coach battery to the toad battery. I suggest you run it through the umbilical between the two, as it's cleaner than a separate cable. Put a 15A fuse or circuit breaker at each end, close to the batteries, to protect against fires should the charge line short somewhere between the batteries.
If you want to go the "full-tilt" route, you can use the charge wire Ford supplies through the two connectors mentioned above. While you're at it, connect all the wires that Thor did not run to the 7-pin connector, not just the charge line. That way you'll be ready to add a trailer electric brake controller at a later time, should you or a future owner want to tow a trailer instead of a toad.
Below, I've laid out three options for rewiring using the two connectors. On your coach, they probably are inside the left frame rail, about six feet from the rear. The connectors may be hard to get to. They are also old, so any attempt to take them apart may break the plastic locking tabs off (happened to me).
1) The wires you want are dead-ended in the harness just aft of the connectors. Unwrap the tape and crimp on wires to the unterminated stubs. Run to your 7-pin connector. Rewrap the wires (and add some loom). Attach clips as needed to support the loom.
2) Ford sells a harness (PART # F7UB−14A678−C) that has the two connectors you need, and a 7-pin round receptacle. The wire between said parts is too short for the receptacle to reach the rear of the coach. Cut off the Ford 7-pin receptacle and discard. Crimp wires to the two connectors long enough to reach your existing 7-pin receptacle location. Wrap the wires in loom. Unlock the existing connectors (here's where you break the tabs by accident) and remove them and their wiring. Attach new connectors and wiring. Hang wiring in loom with clips as needed. Terminate on 7-pin receptacle.
3) A variation on 2: Ford sells the two connectors and pigtails as part of an RV kit (would come with the coach from Ford to the coach builder). These come up on eBay every now and then.
I did option 3 on our 1997 Lazy Daze 27-foot mid-bath. I replaced the factory-supplied 4-pin connector with an "RV ProPlug" mounted on the rear bumper.
I have pictures of the rewiring project- way too many. If you're interested in those kind of details, it'd probably be better if you sent me a private message that includes your e-mail address and outline the information you'd like to know/see.