[PNW Fireguy]

If you are just looking to run the fridge between stops and you currently have the stock two 12v batteries installed the simplest solution in my opinion would be to change out the two 12v batteries for four 6v deep cycle batteries. The resultant amp hour capacity will be increased significantly. If your budget can support it go with AGM vs lead acid. Simple replacement and relatively straitght forward. You could conversely install a small solar charger and one or two panels on the roof. Financially you may be at about the same dollars if you do your homework and don't spend on the high marketing options. But the labor to install will consume more of your time and if you don't have the DIY ability the labor cost can be as much as the equipment.