Looking at your battery box, say you are out camping without the floor jack, it looks like you could stack a couple of 4X4 on each other under the battery box, and lower the jack. Then with the weight on the 4X4, you could remove the pins, and then raise the trailer enough to work on the batteries.
Of course the floor jack is really easy, and will lower it down onto a pair of 4X4's and then you can slide out the jack if you like. . Remember it is well over 300 pounds the way it sits, so careful on your back! LOL
Looks like a great job!
Fred.