3406E fuel mpg droping like a rock

I got this use KW 97' t2000 with a cat 3406e super ten (Yuck)back in oct it did ok on fuel high 5 to low 6 not great but ok for an old truck last few months I have been watching the fuel mileage drop fairly steady at first I thought it might have been from the exhaust leaks had one fixed it next week had a new one all in all I finally replaced all of it. But with no luck getting it back up I have been doing low to mid 5's the past month or so I figured it might be my foot even though been running it the same since day 1 so two weeks ago I had a run from Bloomington,In to Denver CO and I ran it the same as always within 2 mph of the speed limit it got a 4.4 OUCH this week I ran the same run but I slowed down a bunch kept it around 65 even in the big speed limit and it got the same 4.4.
Weird is that I ran all last week and was up about 5.3 or so
Indiana to Bismarck ND a few weeks ago was 5.6 heck I ran some of the best mpg about a month ago at max weight going though Ohio and PA on 70 22 and there's some big hills out there got a 5.8 out of that one. J

ust wondering if I might be missing something that may be causing this drop. Or something I might be able to do to get a bit better
I am not asking to get no 7MPH out of it I know that is not going to happen Just want to get back to the high 5's low 6's. 80% of my loads I am 78000+ off and on I get lucky and have and am lighter.

if It does not get better I will have to go back to the Volvo but it likes to up and die on me out of the blue driving down the road needs a new air dryer kingpins and probably injectors.

 

I thought about the winter blends just did not think it would be that big difference
the heavy on mountains is why I tossed in the info about PA since it has big climbs as well but I guess the long slow up may drain it more than the fast short climbs
Was bad weather last night but I figured the big drop in speed should have countered that I mean I went from running 75 all the way though KS and CO to 50-55 in KS do to weather and 60-65 in CO but roads clear
umm boost was staying pretty close to 15 -20 to maintain speed on slight incline which is what most KS and CO are 20 is about tops for this truck tried to go slight less but it would slow to much on the last 50 miles or so I was able to keep it slightly less than 15 and I started gaining a little bit
was windy as all get out last night
Tire steers 110 drives 100 trailer 95-100 I maintain my truck tires but swap trailers a few times a week so I just make sure they are near 100
I pull a Flatbed this load was cut limestone

But I guess all them combined would drop it all a lot and I have an airline that I cant find out where it goes

 

I started thinking a bit more about it today and I had a water leak last week 8Gal in three days and I knew I had a leak at a valve so we changed it don't seem to be losing it anymore But I have not shut it off do to freezing cold the amount that I lost seems way to much for the little leak I had so I checked again and still got water but noticed it bubbling thinking back I had noticed a couple of times when I started after sitting over the weekend that it lobbed real bad for about 20seconds and blow thick white smoke about one min which might mean a blown head gasket and that would also explain the slight loss of power and fuel dropping even before it got real cold and the winds picked up.
Am I correct in my thinking Or way off base?

I do know I need to get rid of this super ten and get a 13 or 18 put in I hate the spread on the gears I never use 1-2 3-8 are gone before I get to 35 that's 9 35-50 and 10 for everything above 50 and I am pretty sure it came factory with a 13 gear shifter is set up like a 13 but front button does nothing at all.