We're trying to gather as much info as possible. DH is meticulous about battery maintenance. Ours need replacing before next season. Will add solar when we can afford it.
Solar is cheaper than you think. Can add a 200 watt system for under $400. Read my recent thread on
Opinions on 100W Solar. And if you are staying where there is sunshine, will be amazed how much power you will get.
In regards to the jacks, make a pigtail so you can lower the camper while still attached to the truck. Won't pull all the power from the truck, but will pull a good bit.
Back to your question on the tray. If the tray was designed properly, it should not be losing that much space. It may be a simple case of having a new tray made. If you can get two batteries, even Group 29s, you should be able to have several days on the batteries with conservative use.
If you could post a pic of the battery compartment opened and closed, it would be helpful. Depending on the design, it may also be a simple case of putting a piece of plywood in the bottom of the compartment.