Well, I tried it out today. While it wasn't the most pleasant job I've ever done; it could have been a lot worse. So it looks like my readings come out as follows:
1/3 full after about 8 gallons
2/3 full after about 21 gallons
Full at about 35 gallons.
No so far that all jives with my trailer's specs; which can be found here: link
However being the adventurous/ crazy person that I am; I wanted to see how full I could make the tank. I ended up putting in 45 gallons of water to get it to the top of the tank pipe; so I could see it in very bottom of the toilet.
Looking at those specs again; I notice other similar models show a 41 gallon black tank. Given my margin of error of filling 5 gallon buckets, that does seem to be right. And the grey tank does look smaller than the black tank when I look underneath.
So what do you guys think? Is it more likely that my black tank has a lot more room that what the specs say, or is it more likely that the Forest River specs have the tank sizes reversed?