Sounds like you got yourself a nice truck. Now this is only a guess, but I am thinking you are going to be lucky to get 10 towing. I have a 2500HD Crew Cab with the 6.0L and I have never towed a trailer, only a boat, but I don't think I ever did better than 10. It will also depend on how you drive, I noticed a difference between 70mph vs. 62mph.
You will go longer between fill ups, but that is because you will have a bigger tank.
Now I have not always been a fan of tow ratings and what the owners manual will tell you a truck will pull. You not only have to pull the trailer, you have to control it and stop it. Either way it goes you are going to be better off towing with the Silverado vs. the Explorer.