2004 Toyota Sequoia...What can I tow?

I would suggest keeping the trailer under 4000 lbs dry weight and under 24' length, with a dry hitch weight less than 500 lbs. I'm surmising that your receiver will take up to 650 lbs (10% of the tow rating), but check on this to make sure because IMO this will be the biggest limiting factor. I once had a 23' TT with a stated hitch weight of 380, but this doesn't include LP, battery, water, or gear; the 30 gallon fresh water tank was up front and when full I bent a rear spring on my SUV (6000/600 rating). Moreover, you have to pay attention to your SUV's payload capacity because with 4 passengers, some gear in the back, and the weight on the hitch, you might exceed the payload rating.