I was going to ask Dusti if he could send me some plans of those tanks so I could run some numbers on them to see what kind of capacity they would calculate to, however, the more I think about this, the more I think there is no point. That capacity would only be valid with a relatively new tank, and as I and others have mentioned several times above, the tanks degrade over time. I would venture that the vast majority of us who have tanks, have tanks over 10 years old, and many are over 25. There was one mention of a tank from 1965, so almost 50 years old. Not exactly "new". The rate of degradation depends on a lot of different things like the chemistry and moisture of your soil and the chemistry and quantity of what goes into the tank (various caustic cleaners, family of 12 versus a single occupant, etc.). What I am getting at here is that the situation is always changing, so just because you got away with something before does not mean that you will be able to continue to do so. Also, modern tanks have at least two chambers with a dividing wall between. Assuming the dividing wall goes all the way to the lid, it will provide a strong spot there. Maybe one time you won't hit that strong spot?
The main reason I think this: how often have you heard news stories about a horse being rescued from having fallen into a septic tank? I can think of about three in the Central Florida area just over the last year. It never seems to be the enormous draft horses at 2000+ lbs or even the warmbloods at 1500 lbs. It seems like this usually happens to horses who are older and thin. That is probably because they are quiet and therefore allowed to graze outside the normal pastures in places where horses don't normally go. It reduces the mowing and then they don't have to struggle with the younger horses for food. The ones I've seen could not have been over 900 lbs.
My wife was a riding instructor for many years. One of her students had a horse who broke a leg going through a septic tank. This particular horse was actually a Connamara pony, so it would be 800 lbs. tops.
We've all heard stories about various things busting through. That pony was only about half the weight of a Cub in the lightest trim, and he went through. You really can't know what condition the tank is really in unless it is brand-spanking new. Even if the guy who pumped it out last week said it looked great, he doesn't have x-ray vision. That wire mesh in there could be well on its way to turning to red dust, and you'll never know until you break it open. I don't care if you've driven your truck over it dozens of times before, my advice to you is to stay off it! The next time your luck might run out and you go over it in just the wrong spot. The more I think about it, the more I come back to it just being a bad idea.
This is obviously just my opinion, and you are welcome to endanger yourself in any way you see fit. I think it's irresponsible to take a Cub, of which there are a finite number left in the world, along with you, however.
set [rant] = OFF
Seriously, though, most horses are a lot lighter than any Cub, and it is not uncommon for them to fall through. Stay off!