Only addresing original question but replacing just the wire from factory relay to camper, though might be some gain-I wouldnt think all that much or really worth the effort. At least on ours thats what I finally concluded. I was going to replace wire from relay to camper, but unknown convoluted path before it even gets to factory relay (and smallish wire) so I left ours intact. Added 'another' charge source along the lines described by others.
Im using an ign controlled constant duty solenoid, I added a manual switch to disable.
edit on your ford factory realy is in the engine bay attached to fuse box. You need to match ground wire size to power wire size (hence the 2 wires) when upgrading. Batteries wont be overcharge-thats a function of alternator. Theres a couple of threads in the TCU sticky under 'Camper Electrical, 12v batteries/charging/wiring/bed plug' .
