I added one, I removed the tool bag you mentioned from under the driver’s seat and installed a 50 AH lifepo4 lithium battery, I powered it with a Victron dc to dc charger to charge it when needed, around once a month for me. I normally run a auxiliary battery in my vans to power my laptop & printer to print invoices.
This setup works great for me, but when I found a 100 AH battery that fit under the seat I installed it and now only turn on the dc to dc charger around once every two months.
But before I started getting into and learning about lifepo4 batteries I had a AGM battery from Sam’s Club that I ran for a while in the area in back of the driver’s seat, it was pretty big and weighed around 42lbs, with around 50 AH, the 100 AH lithium is around 22lbs. There are many good write up’s on this forum of the different setup’s and batteries & dc to dc chargers that were used, I’ve learned a lot here.