I got the Barker 42 gallon (https://www.amazon.com/Barker-30844-4-Wheeler-Tote-Tank/dp/B000OR18BE/ref=sr_1_1?ie=UTF8&qid=1528248070&sr=8-1&keywords=barker+tank+42+gallon)
A bigger tank can do the same as a small tank and then some for a little more money. Sometimes I use it to just empty black water but mostly I empty black water and then grey water and since I use a flojet macerator to push it into my elevated truck bed, I also put plain water to clean the lines.
I highly recomend that tank and as recommended by people on here, https://www.amazon.com/Flojet-18555-000A-Portable-Macerator-Carrying/dp/B001HKZHIW/ref=sr_1_1?s=automotive&ie=UTF8&qid=1528248321&sr=1-1&keywords=flojet+macerator.