Math isn't mathing

OP you’re tracking correctly. 
However the simple posted capacities for (most any) RV are simplified to reflect the safe load capacity knowing they cant account for all the variables of how it will be loaded. 
But your math is correct. In that if you add full water (huge tank btw), full gas (not a lot of weight) and your roughly 1600lbs of toys you are at the rating almost spot on. With no other gear aboard. 
And a resi fridge so plan on full hookups, 24/7 generator screaming or more batteries for a legit inverter setup. That alone would send me packing from this camper. But maybe you never go where there’s no hookups. 
What isn’t explained is the water will about balance the toys load and if you’re closer to maxing out the tires and axles you will likely be ok.  However you don’t want to do that balancing act nor does anyone. 
Just like other campers different toyhaulers have different ccc numbers. 3600 isn’t bad. Some are over 4klbs and some are considerably less. 
Your math is fine. 
That model is not really suited to hauling a big 4 seater sxs or your 2 toys which weigh about the same as a big 4 seater. Just because it’s a toy hauler doesn’t mean it will haul “every” type of combo of toys. 
PS, based on your fridge type, and my comments regarding that, either you’re looking at the wrong configuration for your use or you will likely have water at the campsite hookup so no need to pack a cistern full of water. Gotta figure in your intended use. 
Lastly, when we had a toyhauler I looked at several when shopping and can say they do a lot to ease the “cracked frames and overloaded axles” concerns that are common with many non toyhauler trailers with very little ccc on paper. The frame on our 2017 Torque was STOUT. I would have had no issue loading full water and a big SxS with respect to the frame. 
Bottom line is you just learned that just because it’s a toyhauler doesn’t mean it can universally haul the heaviest toys you can put in it without exceeding some rated capacity or other. 
Broaden your search and see if you can find one similar with a little more CCC maybe. Although you’ll probably only find around 500lbs more than the camper in question. 
Every single RV is ideal in some scenarios and a compromise in others. Doesn’t matter if it’s a 43’ tag axle pusher or a popup tent trailer or anything in between.