First, let's get it out of the way now so your thread doesn't get clogged with the same comment, Check your local and any state you're traveling in on double towing. Most of the eastern and some of the western seaboard doesn't allow it. Also count on any existing frame warranty will be void after this. You might want to wait until the 2 year frame warranty is up.
O.k. with that said, I built a subframe out of 1/4 thick wall 4x4 square tubing. Then I welded that to the 5ers frame and gusseted it. I did this on 2 5ers, one being my own. I tow a 5x10 utility trailer with my 05 Yamaha G22 car. DO NOT ATTEMPT TO UTILIZE THE FACTORY BUMPER, if yours has one. Make sure your 5er has a substantial frame. Mine is a 10 true I-beam.
Here's a couple of pics from during construction.
The other 2 receivers are for a motorcycle rack that I use on occasion.
The last pic was our first trip with the setup a few years ago to Florida and back to NJ. The pic is a little deceiving but everything sits level normally.
*Current: 2005 Ford F350 Crew Cab Dually 6.0 diesel 4x4*
*Retired: 1987 F350 Crew Cab Dually 6.9 turbo diesel
2007 Forest River WildCat 32QBBS