I was really hoping someone would be able to look at my pics and say, hey, that's the xx gallon tank and save me the hassle of crawling underneath my toy hauler after a hard rain. Well, guess not . . .
So, very relunctantly crawled underneath my toy hauler with my tape measure and did some rough, quick measurements (the suggestion BarryD0706 first made and which I balked at, sorry). The tank has 3 "sections". The front and rear sections are the same size. The middle section is the large section that drops down much lower than the front and rear sections and also has the "indents". "Swagged" the measurements for the wierd-shaped "indents" (as Healeyman suggested) and subtracted this volume from the large middle section.
I came up with 76 gallons. I'm guessing the position of the input hose and vent (on the side, down about 1 - 1 1/2" inches from the top of the tank) probably limits the tank to about 80% of its volume. So, 80% of 76 gallons is 61 gallons.
These figures concur with my realworld capacity of 60-65 gallons. Mystery solved. If the 110 gallon capacity was a real priority, I'd be pissed. But, I don't dry camp that much so I'm ok with the lower capcity. My black (25 gal.) tank typically fills up before my freshwater tank empties (kitchen sink is plumbed to the black tank). Even if I had the 110 gallons I'd still have to pack up and leave
Thanks to everyone!