I have an older Expy, 2005. Short wheelbase, HD tow package, 4x4, WDH for trailer. I towed a 26' 6200# camper with no issues. It did "wag" a little, especially in crosswinds and fast semis passing on the interstate. I keep towing speed down to 60-65 at the most. The first thing I did though was upgrade that ridiculously small rear sway bar! Stock is only 1/2" -- I got a 1.25" bar (might be 1.125"... I forget!). That made a world of difference! I then got a 30' 7200# camper. Yes, I'm pushing the weight rating, 300-500# over when loaded. I don't boondock, so no water in tanks or generator. I added Sumo spring helpers to it shortly after getting the bigger camper. Squats about an inch less... I think. I can't really tell any difference in handling, but I only take the camper out 3-4 times a year. Most trips are under three hours each way. Took it on a 4.5 hour trip to the NC mountains once and had some pretty long grades. Speed dropped to about 45 going up the longest, but it stayed in third gear (four speed auto) and didn't drop to second. Would have had I pushed it, but I was in no hurry. Engine temp stayed good, and there were no issues at all. I will add that I had the trans rebuilt a few years ago due to a failed snap ring (lost 3rd and 4th... OD). It was rebuilt with all the upgrades for towing. I haven't upgraded the factory trans cooler. If I were pulling more often, especially in the mountains, I would put a larger trans cooler on it. With the heavy load that's the only thing extra I'd do. I've investigated a "chip" for the engine, but the only reasonably priced ones (under $300) are "learning" types. Since I drive the thing more without the trailer than with, those would be pretty useless, as they all say they learn after 2-3 days of driving.
That said, I'll add that anything over 26' tows differently. I went from a 24' trailer to the 26' and noticed no difference. The longer 30' was VERY noticeable! Two more feet between hitch and first axle made a big difference in turning radius. The two extra feet behind the axles means a bit more swing, but that wasn't much of a problem. I've told my wife that the only way we get another camper is if it's 26" or less. Just took a little more to learn to maneuver it, but it does mean you need a lot more room to move it around. My 30' is a V-lite with three slides -- two opposing in rear, one for the bed in front.