OP is running a Yeti-style battery pack.
I charge a 20,000mAh external battery
Onlookers: 20,000mAh = 20Ah.
I'm out of the Van most of the day and charge that battery while I'm working
That is a perfect use case for such devices. [not joking]
I do not yet own a refrigerator, but do plan to have a fridge and full house battery set up with AGM batteries within a few months hooked up with a solenoid to the alternator for charging. No solar panels for me as I'm stealth camping in New York City.
Something to consider when planning out the future setup: it will be tough to keep lead-acid batteries (including AGM) healthy with alternator charging alone. Possible workarounds:
budget to replace lead-acid batteries more often. Give real consideration to flooded batteries; they are half the price and more resilient to abuse.
Normally I would encourage to a ‘dweller to grit their teeth and install a panel or two: the panel will provide high voltage, long duration charging to complement the high current, short duration charging from the alternator. BUT stealthing in NYC brings a special challenge – tall buildings make “urban canyons” that make direct sunlight a rarer occurrence than elsewhere.
Because of the urban canyon effect, I’d encourage you to think about switching to a battery chemistry that doesn’t care about state of charge. Both lithium and carbon-foam (a subtype of AGM) are much more tolerant about infrequent or incomplete charging.