Took the plunge and traded my Excursion for an Expy EL... love it so far (but miss some of the space). In an earlier post I asked what people thought about their towing experiences with the EL, and now that I have one with the 18" wheels I've got a concern...
My rig has the heavy duty tow package and is rated at 8750lbs (subjective, I know). My concern stems from the original equipment tires (255/70-18 Pirelli STR A, P-Metric) and the fact that I've got a 7000lb 30' travel trailer to pull on those tires. With 2470lb load capacity on each corner and a rig that weighs, say, 6250, plus 500lbs of gear/people/dogs, plus 800lbs tongue weight I'm under the GVWR / GCWR / max tongue weight (w/ equilizer and sway hitch), etc... and I'm within the load carrying capability of the tires, but am I going to have a wishy-washy dangerous mess going down the road on these tires? Its not the weight I'm worried about necessarily, its how well the sidewalls will hold up to the heavy side-to-side forces. Long, tall trailers can even be a challenge for the bigger Excursion with E-load rated tires.
I've got a 08 Expedition EL 4x4 w/ the 18s as well, I went from a V-10 Excusion (and 8.1L Yukon) and I have to say the expy is 100% better all around. I believe it tows better and stops MUCH better. I get better fuel economy w/ the expy and the same trailer weight and i'd say I have the same if not better power due to the 6 speed. I've towing 9500 lbs for a while without a problem, the temp guage doesn't even move in the FL heat. I'll be going to 35" BFG ATs when these wear out but for the time being the OE tires work GREAT. Only option I have you maynot is the rear air ride, it makes a huge difference towing. I also use a weight dist bar w/ sway control.
Is your trailer #7,000 empy or loaded and have you weighed it? You might find you're heavier then you think.
I've towed about #5,500 two times now between Pheonix & San Deigo without any problems, and that is up and down some pretty steep grades. I have an 07EL 4X4 with the same tires. My trailer is 30', but is a trail cruiser 30QBSS so it is much shorter in height (perhaps 2 feet) then most trailers. Its frame is much lower and has the torsion axles instead of the normal leaf springs which help to reduce the height and increase stability. On my last trip 2 weeks ago we had at least 40 MPH cross winds and head winds and I had no sway driving at 55 MPH (mileage sucked at 9 MPG!). I have the Reese Dual Cam Sway Control and couldn't be happier.
I don't know the axle ratings of the Expy, but you might be getting close to the rear axle and tire capacity... Best to weigh.