Very curious as to which Ford van you're using. As suggested above, if you can get a manual (start by searching on line) for your year, you can find out the towing limits. If your van can handle the loaded weight of your 28', you'll be just fine. And that mileage is just incredible for towing, even at Calif's max 55 mph. Our Sprinter gets around 14.5-14.7mpg towing at that speed with its small diesel.
Our 2009 28' International is 6100 empty. When loaded for the road, with pots n pans, dishes, full propane tanks, a 1/4 tank of fresh water, groceries, drinking water, clothing, tools, etc, it weighs 6980 lbs (that includes 130 lbs of two extra batteries). So I would guess that you're closer to that weight than to 6000 lbs, and I'd use 7000 to figure what you're actually towing, just to be safe (I believe that you are also rated for the AS to weigh a max of 7300lbs).
Which engine and differential are in your Ford? They often comes with a smaller V8 and a very large differential to make them more powerful for towing, but this usually results in poor gas mileage, too!