In my personal opinion and experience, the good ole SLA battery mentioned in the Original Post will likely be the best route.
I’ve had good luck with $20 7Ah SLA’s with a common $35 12V Solar Panel & Controller.
Use a full size battery box that has lots of room for insulation and the solar panel mounts on top of the box .
Lead acid batteries work fine in North Dakota, every vehicle has one 
They are hard to beat when space and weight are not a design constraint.