So, I looked at 29DC marine batteries. $100 each. I can fit only 2 of them.
walmart.com/ip/EverStart-Maxx-Group-Size-29-Marine-Battery/20531539

But having a brain freeze on how to wire them for both 24V AND 12V since I need the 12V for charging purpose. The battery pack assy has to be lifted out to get to them.

Unless someone can come up with a drawing......
The other down side is there will not be enough amps to safely run the jack. Not sure about the Ah of the Everstart but looks like about 120Ah. The application is barely an hour a day usage. If I can get the wiring worked out that is the route I will likely go and take a chance the Ah will be OK.

$200 dollars is much better than $840.

Here is the spec sheet for the UB62000:
powerstridebattery.com/sidebar/45969.pdf

The on-board charger is:
Cust Model: SVR2425125/6
Model: SVR24251205/6
Not sure what the zero difference is.

Yes, I'm over analyzing this. Just see what I can do if I had a pot of coffee right now.