This is a tricky question because all of these RTTs are different.
For ours, we keep sleeping bags, pillows, the foam mattress that comes with the tent, the window poles (the metal poles with the hooks in the end), and a collapsable Ikea box for shoes, all in the tent when it's folded up without any issues.
You shouldn't have had a problem with the air mattress up there, depending on the mattress and the thickness of your sleeping bags. Down-filled, synthetic Backpacker sleeping bags will compress a lot more than the heavy canvas ones will.
The real limiting factor with stuff inside the tent is best checked at the hinges. When you fold it, the hinges are 'Fixed' width and so anything wider than that width would be putting undue pressure on the mechanism. I'll see if I can draw a picture to show you what I mean.
That being said, we found the Smittybilt velcro and straps to be crap. We had to replace all of ours, so if you find yours aren't very sturdy it could be an issue of strap quality, not stuffing of the tent. Home Depot has tons of cam-lock straps that are way better than the Velcro/D-Link straps, and honestly it's such a handy upgrade I recommend it even for nicer tents. The cam locks are easier to use when wet, get tighter, and hold way more securely, and are not impacted by mud and grime nearly as severely.
Edit: Here is the picture that I'm talking about; your limiting gap is at the hinge end since those hinges are fixed and without flex, so as long as the end opposite the hinges is not "alligatoring", you should be OK. Of course, things compress and squish, but if I can't get the two halves even, and avoid that 'alligatoring', using the velcro straps (before I put the cover on) I take stuff out until it is even. As I said, the SB velcro is really terrible and you might need to replace it, but it's only about $10 worth of stuff to do that job, and then this principle of 'anti-alligatoring' will apply and not do any long-term harm to your tent (at least, not that I've noticed in the 7+ years we've been using various RTTs).