I'm upgrading and just bought a lithium battery for mine. It's a "drop-in" lithium which means it's internal "Battery Management System" is designed to work ok with a lead-acid oriented system.
I did a lot of research on drop-in LiFePo4 batteries (the type to use) and finally settled on the 100 ah "Battle Born Battery".
Both "Bay Marine Supply" and "Northern Az Wind and solar" sell it and both currently offer a discount over the US-based mfr's price, though you do have to ask Bay Marine for the discount. Bay Marine is a GREAT company to deal with, by the way, very helpful and the best prices around. Nothing against NAWS either, good folks.
100 AH might not sound like any great advantage considering the high initial cost ($850 shipped with discount), but these lithiums should give much more real usage for the AH rating, size and weight.
- you can draw lifepo4 batteries down all the way if you want with little issue, so 100 ah lifepo4 has more useable power than 200 ah lead. Most sources indicate that 100% DOD is not an issue when needed, but that 80% DOD will give the most cycles
- they hold their voltage up until the very end, so your fridge won't be cutting out early.
- they charge much faster to 100% and will soak up more amps when available, so smaller panels or shorter drives will still get you your full capacity back sooner. You don't have to "spoon-feed" them juice over long periods like the AGMs and other lead based batteries.
- unlike lead, they don't care if they don't get fully charged.
- minimal self-discharge
-longer life in storage
-many more cycles
Caveats: Just like lead, keep 'em cool to make 'em last. Don't charge when the the battery is frozen. Internal electronics on the Battle Born take care of the freeze factor. Mounting inside van keeps 'em cool.
Extensive comparison on value/capacity of lithiums vs AGM
https://www.victronenergy.com/blog/2015/03/30/batteries-lithium-ion-vs-agm/I'm doing a solar setup as well myself, which I'll detail once I get it installed, but thought I would give a heads up now for anybody who is about to buy and unaware that lithium is maturing as an available tech and seems especially appropriate for smaller vehicles where small and light really do count. IF you have the $$$ for initial outlay, they might actually be more economical over the years.
From everything I have seen, these will probably give a long enough run-time for a fridge with no solar, just alternator charging if you are not boondocking for long periods. And though the alternator should charge the "drop-ins" decently as-is, because lithiums like different voltage levels than lead and mainly, no float charge, it seems that changing the relay over to something designed to work with lithium will get the longest life. The Victron Cyrix Li-charge is the one I'll using as recommended by Alan at Bay Marine:
https://baymarinesupply.com/victron-cyrix-li-intelligent-battery-combiners-for-lithium-systems.html I had originally ordered the Li-CT, but he called and convinced me to go with the Li-Charge as it's uni-directional and less issues than the bi-directional Li-CT
cheers