here is the data for a 1984 model and i'm sure there is no difference to the 1985 model year.
front axle wt: 2250 lbs. it could have the snow plow pkg with 2750 lbs, but i doubt it as those are relatively rare.
rear axle wt: 2598 lbs
now i know that totals 4848 lbs gross, however the Ford Specifications Book i'm quoting from lists the GVWR at 4480 lbs. don't know why the difference, but there it is, and that is the maximum weight rating for your truck. if you wiegh your truck at a local weigh station, you'll probably get a weight of around 3400-3600 lbs or so leaving you with about 800-900 lbs for you, passengers, fuels, and stuff in the back PLUS the weight on the hitch ball. your step bumper is rated for 2k lbs with a ball on it, but i sure wouldn't try it. using the frame mounted hitch, i would still keep the trailer to no more than 2500 lbs and feel much safer. that means that 250 lbs should be on the ball (10% of the towed weight on the ball for correct towing). i don't know how much the trailers you referenced weigh, but i'm guessing more than 2,500 lbs.
additionally, look real hard at the frontal area of those trailers and see how much wind resistance they are going to give you. if you live in Kansas, ie: no hills-really flat, then you might be ok if the winds aren't too strong. most anywhere else in the country where there are hills, mountains, etc. and you're going to have problems i believe. no offense to Kansas as i used to live there.
so, on a long trip pulling one of those trailers, i'm guessing you'll get somewhere around 8-12 mpg's, maybe. with the 17.5 gal tank, you'll be refueling roughly every 175 miles or so. every 150 miles or so if you only have the 15 gal tank. man, that would make for a long trip in my book, but that's just me.
hope i answered your questions to your satisfaction. good luck and welcome to TRS.