You calling my wife and I fat?? J/K
If you're referring to the factory roof rack load capacity, keep in mind that first off that figure takes in to account the vehicle moving, so 300 lbs atop a moving vehicle is exerting much more than 300 lbs of force as the vehicle sways and rocks. Stationary it can support much more as a static load. The other part is that an aftermarket roof rack will do a much better job of distributing the weight than the factory cross rails. I don't have very much science to back it up, but there are probably at least 100 people on T4R with a RTT on their 3rd gen and their roof hasn't caved in yet. Much less the hundreds more in other generations which probably have similar listed load capacities. I just know it's worked for countless others and you can't beat the comfort of that mattress.