Yep, that's too much weight for a Navigator. But even if the trailer GVWR was 6000 or less, you'll still burn up the tranny without some expensive mods to the SUV.
Mainly, you need vastly increased tranny cooling capacity. Replace the radiator with the OEM super-cooling radiator which has the oil-to-water heat exchanger in the bottom of the radiator, Then add a big oil-to-air heat exchanger in front of the radiator.
Then if your SUV doesn't already have a good digital tranny temp gauge as part of the dash display screen, you need to add a tranny temp gauge with the sender in the sump (tranny pan). That means adding a threaded bung in the side of the pan so the ATF covers the sender. Then use 225° as your red line for tranny temp. If your SUV has the analog-looking tranny temp gauge on the dash, that's not good enough. It's not analog, but an idiot gauge. It jumps from the white or green zone into the yellow zone, and Ford's weird definition of yellow is STOP an cool off the tranny. So you'd have to keep an eye on that gauge and stop the instant it jumps into the yellow zone. I know I can't watch my gauge that closely, and I'll bet you can't either. So you need to install a real gauge with clear easy-to-read markings to indicate 225° or else a digital gauge.
Of course you also need the trailer towing wiring harness with a 7-pin RV plug on the rear bumper, the trailer brake controller connection under the dash, as well as a class III/IV receiver. Class III means up to 500 pounds weight-carrying (WC) tongue weight (TW) capacity, and class IV means up to 1,000 pounds weight-distributing (WD) tongue weight capacity. And yes, you must have a WD hitch for any RV trailer with TW more than 500 pounds or gross trailer weight more than about 3,850 pounds.
If a Ford integrated trailer brake controller (ITBC) was available for your year of SUV, then you need to add that too. It's much better than any aftermarket brake controller.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.