We tow with a 03 and a 07 Expy. Our trailer currently is only 4200 lbs dry and well within the Expys specs. On the open flat road I tend to forget the trailer is back there at times but not in the Rockies. Then again in 30 years of towing in the Rockies, I have never owned a trailer truck combo that didn't know we were climbing a steep grade like Crow hill.
Personally I would sty around 5000 lb dry, maybe a bit more depending on how the tongue weight worked out. It's not always the same trailer to trailer and like with mine, the fresh water tank is in the front. Filled it adds a lot to the tongue. On the slide version of my trailer the tank is back towards the axles and has much less effect when filled.
You payload is a bit on the low side for a Expy, so make sure you account for what you are doing. You will have abit less than 600 lb to work with if you max out the tongue, out of that has to include passengers, gas, the hitch and anything you put in it. The nicer hitches weigh more.
You will have to have a WDH to get to that weigh, make sure you get sway control too.
Last thing is if you do this, the next time you get tires get a good set of LT tires. The P rated tires are squishier to give ride.
