I have a 30 gal black water and 30 gal gray water and found a used 12 gal portable tank on craigslist for $30. I never transport black water in it, just gray. I fab'd a mount with ratcheting cam straps for the underside of my trailer. The tank gets placed on the straps upside down, then cam'd up securely to the rear belly of my trailer. Been that way for 2 seasons and have had no issues. The hitch mount is a good option if you don't want to crawl a bit to get to the tank, but I have no issues seeing I use that thing maybe 4 times a year.