You are right to be concerned with carrying capacity. If you can find a high quality unit, you will also notice they are heavier empty than less expensive units. That family of six will use up that carrying capacity quite quickly to the point where carrying water as you travel, or gray or black contents, becomes an issue.
Slides also use up carrying capacity too.
Also be wary of the wheelbase and notice the differences between units. A longer wheelbase is necessary to help with carrying the load and with steering.
Check out the Winnebago Outlook 31N. Winnebago puts a lot of nice extras into their units, definitely worth looking at if you can find one.