I bought a used trailer a while ago. It had a hydraulic jack with two batteries.  I'm having problems with the batteries allways being drained.  I had kinda thought it was just because they were 3 years old.  But I started to think, I'm not sure they are getting charged other than when I put a charger on them at home.

My past LQ always charged the batteries through the truck connection or generator,  My trailer before charged the batteries off the trucjk as I drove.

So I guess I need to tear into this trailer and see if it gets any charge from the truck while I drive and if not, find a way to make that happen.

From what I can see The two batteries each have a ground to the trailer frame.  The jack takes one red lead off battery # 1  The trailer takes a lead off battery #1 to the pos post on battery #2 and then through a fuse and inside to the lights. ( DR, Horse compartment and load lights)   The wiring harness from the truck 7pin connection, runs into the frame and I assume to the brakes and tail lights.  I see no obvious connection back into the battery.  The lights do not work (regardless if the truck is hooked up and running or not) if the battery is dead.

So what is the right way to charge from the truck while driving?

Since the interior lights don't work from the truck if the trailer battery is dead... tells me either the truck is NOT supplying power or the wire size/truck power is too small to charge the batteries and lights.

I think you'll have a hard time driving enough time for the truck to charge two batteries to your needs.  You would be better served by having a charger on your trailer charge the batteries from utility power.  Then your batteries can get an equalizing charge.  And a longer life.

Another thought is: the trailer batteries are self-discharging between the two batteries.  Is there an isolator between the batteries?  A pair of 6 volt batteries in series is better than two 12 volt in parallel for this reason.

No Isolator, Just a jumper wire from Pos to Pos post and a common ground for the two Negative post.

The truck is putting out power.  It charges the batteries on my other trailers.  Dump trailer in particular, has a battery to run the hydaulic ram that dumps the trailer.  It's a single 12v battery but charges totally off my truck.  And it gets much shorter drive time between uses.

One thought would be to pull the load lead off the battery, feeding the lighting and jack circuits. You may have a partially grounding circuit that is draining your batteries. This can be verified by placing a meter in series with those circuits when everything is turned off. If a load is detected, you have a grounding source.

Also with the B+ cable. If you check your truck's source feed, you should have a 13.8 voltage reading with the truck running. If you don't have this, then obviously, the batteries are not being charged. You can then check the trailer wiring to the plug for an open feed.

gard