Well first off welcome...
ONE THING YOU SHOULD REALLY BANK ON IS 4WD!!! One of our other members only has front wheel drive and had some minor traction issues trying to pull a boat out of the water, but you'll be hauling a trailer and I would apply the same situation to you when going up hill or for emergency. When you get the 4WD system you are allotted a weight increase to a total of 10125lbs of GCVW... Vehicle, Payload, and Trailer= 10125lbs...
You don't get any additional capacity by going with the 4WD. The additional weight in the GCVWR is simply to cover the additional weight of the 4WD components, so the MAX tow rating can remain the same.
As far as the weight of camper:
10125 (GCVWR) - 4600 (approx base curb wgt) - 150 (standard driver allowance) = 5075 lbs
Get a more accurate actual empty weight if possible door sticker
Let's say an actual vehicle with 4WD and other options is 5000 lbs
10125-5000-150 = 4975
As you can see, I'm almost exactly at the stated MAX towing rating. That is how Ford handles the numbers of all their vehicles.
Therefore, any passenger or cargo stuff that goes into the Explorer must be subtracted from that MAX 5000 rating, to get an actual rating.
So if you have 400 lbs of spouse and kids, plus 100 lbs of snacks and DVD players, that leaves you with 4500 lbs for the loaded trailer.
If for some reason your Explorer is decked out and weights 5200 lbs, you'll only have 4300 lbs for the loaded trailer.
So, just do the math, with all known weights. Estimate conservatively for all unknown weights.
DO NOT TOW OVER ANY RATINGS, EVEN IF DOZENS OF CHATSITE MEMBERS TELL YOU IT'S OK.