F350 losing power on hills, can't go over 20mph uphill
 

have 99 F350 Diesel, I am pulling a trailer 4k trailer, thought I had water in my diesel fuel got new air filter, truck ran good for a bit, I am driving from FL to NY, got stuck in PA last night because truck ran good on flats, but when incline came RPM's rose to 3k then decreased, could only go 20-25 mph on the hill. I am waiting at mechanic, but they seem to have no clue what it is......any help would be greatly appreciated.
email is
thereynoldsfamily@hvc.rr.com

you might have better luck posting this on the f350 forum.