My 2013 F250 works similar losing about 3-4mph but it had the 3.31 rear gears, from what I read it's less of an issue with the 3.55's. It's not a preoblem with your truck, it's more of a situation where you are running at a slightly less than optimal RPM for the power curve. I've driven plenty of vehicles over the years and it seems with lower gears of today it's more of an issue on cruise. The worst offender I found was the previous generation Suburbans with the 5.3 and I believe 3.08 rear gears ( I could be wrong on the ratio) they were a dog on cruise and would drop nearly 8-10mph in the hills. We always end up with one as a rental on our family winter vacation it seems, 6 years and counting!
In my case when I run about 60-62 on cruise I lose about 3-4mph, if I run about 65-68 I lose 1-2, at slower speeds I simply would put my foot down to cause it to downshift right before the hill.
