need advice from professional trucker or mechanic regarding truck purchase

i have 2007 volvo day cab with 424k miles, volvo engine and 10 speed tranny ...have no idea on rears , 400 horses , super single , 22.5 low pro fronts.
i am running ez4 kaufman trailer , loaded with 2 big ### suv and 2 regular cars i am getting 8mpg avarage running chicago-MO-KS-denver-NE-MN-WI-chicago , sometimes chicago-denver-NM-chicago....i do only my fourth trip , trailer itself is 9k lb , i guess each car is average 4k lb
my best was 8.7 , worst 7.5 after hand calculating after each fill up
i usually go on cruise control at 61 (previous owner has ecu programmed not to exceed this speed i guess) , so i dont have much experience driving big rigs , but when i go uphill sometimes i have speed drop to 45-50 steep ones , switch to 9 ...slowly but pulling ...not strong at all - may be i am doing something wrong , please correct me or advise.
so it is relatively light load i guess
i hope to get my 2004 7 car trailer from shop after sandblasting and painting , i hope it is going to happen - been waiting for a long , gave deposit on trailer 2 month ago , been promised 3-4 weeks.
i am planning on getting big rig with sleeper
my dispatch is advising me getting 2004 volvo with volvo 435 engine and freedomline auto , i like auto but all pros says no
thinking of getting maybe something with cummins n14 , after awhile overhaul it if needed
i have friend with 2000 pete and c12 inside , he did overhaul himself and claiming his truck doing about 6.5-7 mpg before overhaul
i was reading about cats being very strong but very expensive in repairs and not good on mpg
i was reading somewhere on this forum (dont remember the name) , the owner of newer pete i think 2007 with cat was doing somthing in 9ish mpg
should i stay away from egr systems , is it easy to eliminate them?
ok guys i pretty much told you all the story what i need truck for , just need advice from people with knowledge and experience , i consider myself good easy driver
the only comparison i can make is 2010 ford f350 vs 2007 daycab volvo:
verdict :
ford before custom tune and dpf delete was pretty much like volvo but 6mpg firm no matter what , with wind sometimes 5 , after tune i got about 500ish hp , ford was pulling so strong , i was accelerating even going steep uphill from 62-63 to 70-75 very strong - no ######## here-
smoking every rig with the load i listed before.... but after 7k miles at 70k miles the engine knocked and this pece of junk is just sitting on the yard and waiting until i decide what to do with it.
price range 30-35k cash , no credit is available.
cheaper is better
appreciate all advices , just try to avoid costly mistakes
mpg is one of the concerns

 

the member name frenzy who gets over 9mpg...my bad not 2007 , but 2000 truck

 

IE -thats international eagle right?
what mileage is too much?