Get a VOM and check between pins 1 and 4 on the truck socket ignition on. This will check both sides of 12 volts coming from the truck, note voltage. If that's good... plug in the umbilical cord and unplug your 5er batteries both sides. Now check the voltage at the trailer battery cables. If you get 0 reconnect the negative trailer cable only and check battery voltage between ground on trailer and the + cable thats still unplugged from the trailer It should read near or at the voltage noted at truck socket. IF no, you have either a problem with the ground or the charge line in the trailer or in the trailer umbilical. On all my 5ers there was some kind of circuit protection at the trailer on the charge line. In a Jayco 5er it was an in line fuse at the battery. In my current 5er there is a circuit breaker under the front cover by the hitch. Remember both + and - must have a complete circuit between truck and trailer to complete the circuit.
In my tow vehicle I always add a # 8 wire from the stud on the alternator through a 40 amp fuse and onto a continuous duty starter relay pole, The other side of the relay a continuous #8 wire to the 5er socket. I do the same for the ground side of the socket too. I also generally add 4/0 cables from the neg side of truck battery to the frame, engine, and body, under the hood. Most tow vehicles are woefully undersized for charge lines. My 5er has 8 6 volt golf cart batteries. A miserable 12 gauge wire cant handle either the amps or the voltage drop over 50 feet and who even knows how the grounds are connected. I also add a # 8 wire from the umbilical plug lug in the 5er right back to the + lead on the trailer batteries with a 40 amp fuse there too. Now some here will say thats too big a wire because the trailer plug cant handle the load through its contacts. I can say Ive never had a problem with either may trailer batteries not charging or my plugs. That said, I always checking battery health, connections and making sure all is working.
Let us know what you find.
-Paul R. Haller-