And that holds true no matter how little the toilet mfr may CLAIM it can get by with...a little less for urination only...a bit more for solid waste. The average adult uses the toilet 5x/24 hours, which comes to between 2.5 and 3 gal/person/day. So your 6 gallon tank holds 10-12 flushes...barely enough capacity to last two adults 24 hours on a hook unless you spend all day ashore.

Methinks you might want to spend a little time with Ronco Plastics catalog. http://www.ronco-plastics.net/marinetanks.html They're just up the road from you in Tustin and make TOP quality thick-walled rotomolded water and waste tanks for a very reasonable price (that's even more reasonable if you order through the sbo.com online store)...and have more than 400 shapes and sizes, over 100 of which are non rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank.

If you'd like some help spec'ing it out, give me a shout via email.