Under $100 including shipping: https://www.amazon.com/Class-Customs-Gallon-Holding-WT-2400/dp/B07NLKXK42/

20 gallons is kind of teeny, no? I could probably fill that after one night out. I have a 40 gallon (haven't installed it yet) but it's only meant for maybe a couple of days of miserly water usage without a shore hookup.