So, just my input/observation, based on what you've posted and what additional research I've quickly done.
1) Lincoln Navigator L, 4WD, appears to have a wheel base of 131". If I recall, 119" wheel base would accommodate a 20ft TT. Every 4" additional wheel base would increase TT length by 1ft. That seems to be the generally accepted formula. So, 131 - 119 = 12 inches additional wheelbase, which would accommodate 3 additional ft of trailer. Puts you at around 23 ft. A few feet short of your Airstream 27'. You have to decide how much you buy into that formula.
2) The 790 lb tongue weight you cite, is 13% of dry weight of 5800 lbs. As you add "stuff" to your trailer, you need to put "stuff" in to increase tongue weight to maintain the 10-15%. Assuming your loaded weight of the trailer will be 6300-6500lbs, might be light. You will be surprised how much weight adds up quickly. But, lets assume 6500 lbs, that means your tongue weight should be 845lbs (assuming you maintain the 13% ratio). The WD hitch itself, you should estimate around 100 lbs. So, 945 lbs there at the tongue, although some will obviously be distributed forward and back. You are setting yourself up for a problem if you put all your stuff you add to the trailer in the back of the camper, behind the trailer axles. So, you need to make sure your cargo capacity accommodates your family weight, to include the kids and the dog, the 945 lbs, and anything else you toss in the vehicle. Don't think cargo capacity doesn't matter because the WD hitch puts some weight on the front wheels. Cargo capacity (aka payload)is the capacity of the whole vehicle.
3) While it is true, the WD hitch puts some weight on the front axles, you have to be aware of how much it moves forward, because you could easily get to max, or over your GAWR (Gross Axle Weight Rating) for the front axle. Another limiting factor.
4) As has been pointed out numerous times, one typically exceeds cargo capacity before you reach max trailer capacity. It's a physical fact, and not something that should be ignored.
Now, I will say that I am relatively new to the 21st century RV experience. I went through all this, very methodically, when I was making my decisions. I had been camping and towing 30 years ago, and did not pay attention like I do now. As I was learning this myself, I miscalculated some things. I bought my truck first. Thought I made the right decision. Then, when I was actually zeroing in on the TT, I realized I totally forgot about the most important factor.. CARGO CAPACITY. I cannot see the cargo capacity of the Navigator online, so have no idea. BUT, I had to reset myself on my trailer selection, or get rid of my truck. Fortunately, for me, persistence prevailed, and I found a great trailer that fit ALL my numbers.. GAWR Front and Rear, GVWR, GCWR, Cargo Capacity etc...
I personally wanted to stay well within the margins, because I didn't want to worry if, on the road, I decided to get lunch to go, and have the Big Mac put my truck over the weight restriction... Okay, so maybe that's a bit exaggerated, but only you can decide how close (or over) to the margins you are comfortable with.
Im not saying this combo won't work. Do the math, don't overlook any aspect of the math. It's a matter of facts, not guessing or relying on anyone else to justify what you want to do. Remember, your safety and enjoyment, as well as your families, depends on it.
Mike
Mike
