Looking at buying a portable black tank - suggestions?

Pee in the RV, the other in the bath house. Don't use a gallon of water to flush. You can go a long time on the black tank doing that. Dump the black as needed and on those days dump it first. I usually dump the grays with the tote every third day and can get both gray tanks with one 35 gallon tote load.
Disregard the above if traveling with a teenage girl. You need FHUs with that.