Can you take some pictures of your tongue. Most trailer tongues are pretty standard on width and length so I'm having a hard time seeing why you can't put 2 batteries on it?
That being said; your best bang for space is lead and the best place to get lead is from batteries. So can you put 2 or 3 more batteries in the pass through? You will have to build a slide out tray to do this but it would be worth it. The plus side of this is you will have a lot of battery storage if you boondock.
The next bang for your buck is water. So if you drink beer or bottled water the place to store it is the front pass through.
The next bang for you buck is take weight off of the rear and put it up front. I don't know what kind of storage you have inside up front but take all of the heavy stuff out of the rear and put it up front if you have storage up front or space up front.
I agree with your tongue weight problem. That is WAY to light on the tongue. The closer you get to 15% the better.