You have a 2005 MH so most probably have 7 pin connector with one pin carrying a fused 12 current. All that has to be done is to pick up mating pin on towed car connector (6 or 7 pin - not 4 pin) then run a #12 wire to from towed connector to battery, add an in line 20amp fuse then connect to positive terminal of battery. Material cost is about $1 for wire, about $4 for fuse holder and fuse, negligible for terminal on end of wire, about $3 for wire loom to protect wire from chafing and about half hour labor at most.
If you don't have 12 v current pin on your motorhome trailer plug (that is hot only when ignition switch is in on position) then add wire from your coach battery to one of the empty pins on the connector, 20 amp in line fuse and about 1/2 hour labor.