To get to the OP actual question (info about 2009 model Explorer towing):
The 2009 Explorer/Mountaineer was the 4th generation, which actually ran from 2006-2010 model years. So opinions from anyone in that range will have direct application to your 2009.
I have a 2006 Mountaineer V8-AWD with the factory tow package, good for 7000 lbs max tow rating, and 12,000 lbs GCVWR. This is our 3rd season using it to tow a 2004 Surveyor 261T hybrid (front tipout, rear quad bunkhouse, 27ft). I'm embarrassed to say I have not had the camper weighed. Sticker dry weight is 3980lbs. By typical standards (1000 lbs over dry weight), that's 5000 lbs. Maybe a bit more for a long trip, when we have all 5 bikes loaded in the bunkhouse, and maybe more stuff in the tanks. I just added about 300 lbs of steel and wood in my floor repair/replace project (see thread in Repair section). So I may hit 5500 this year (need to find a CAT scale in route this year!). It's pretty much just people in the truck, and a little bit of firewood in a plastic tote. So ~5600 lbs loaded Mounty. That's using around 11,000 of my 12,000 lbs GCVWR.
I am very satisfied with our Mounty's towing ability. The 300 ft-lbs and super-low 1st gear in the 6-speed auto gets the rig going well. I use a Scangage to monitor fluid temps and torque convertor lock status. Prodigy brake controller runs thru the Ford specific pigtail to the socket under the dash. We did a 2-week pull around the perimeter of Michigan's Upper Peninsula last year, and the driving was never stressful. Get a good WDH like the Equil-i-zer.
Compared to an Expedition pulling a full-height camper, the Explorer looks small, a bit mis-matched. But they didn't give it a 7000 lbs rating for nothing. If an Expedition would fit in my garage during snowy Michigan winters, I'd probably have one. But then, I'd probably have a heavier trailer, too. For the amount of camping that we do, and the moderate terrain we cover, an Explorer/Mountaineer (2006-2010) is an extremely capable tow vehicle.
If your Explorer does not have the factory tow package, you should not tow more than a 3000 lbs popup or utility trailer (3500 max tow, 8500 lbs GCVWR), but nothing with full front area.
If you have the V6 with tow package (5000 max tow, 10,000 GCVWR), think 17-19ft ultralight (Roo, etc). But the full frontal area will be taxing at highway speeds on the V6 drivetrain.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller