Need help ASAP 94 Isuzu trooper very low power and only 5 mpg

Okay so I have a 94 Isuzu trooper 3.2 dohc
that I just bought and me and my father
got very short notice about a job so we're
on the way to Idaho from Louisiana, the
vehicle idles at about 800 rpm but jumps to
900-1000 and then back down, it has very
little power and is having a lot of trouble
going up inclines and you have to pretty
much keep it on the floor to keep it at 75
on the freeway, it keeps shifting from 5th
back down to 4th, and is only getting
5mpg, since I bought it I have replaced the
spark plugs and wires, changed the oil and
fuel filter and cleaned the egr valve, I have
also noticed there is power steering fluid
on the alternator which in a couple cases I
have read that could cause these issues
but do not see how that's possible, please
anybody if you have any suggestions let
me know, need to have this fixed ASAP so
we don't get stranded somewhere hours
from home on the side of the freeway