Looking to buy

I'm looking on buying a newer truck I have a 2009 international prostar 485 cummins 13 spd 355 rears. When the truck was built it was built to haul a 3 axle trailer with papper. It weighs almost 24k with my rack chains,binders APU and 2 tool boxes....my bad for not weighing the truck when I bought it and before I stretched the frame...
It only gets 5 mpg and I have done everything to improve that....she has 780k on it and oil samples come back clean.
That being said I need to buy a lighter truck that gets better fuel millage.
I'm looking at cascadias what should I buy
I'm thinking I want a 13 speed stick?????
I pull a flat out of Boston as far as Texas and South Dakota....no west coast
Looking for susjestions

 

Been there and done that . Changed all sensors,actuators, injectors, slowed down to 60 only gained .05 . Full fairings skyrise sleeper and I drive like a old lady never in a hurry or first up the hill. I have had prime pass me...

 

That's all fine but it weigh almost 24k and I can only scale 42k on a all aluminum flat that leaves me limited in the available loads. And the 3 factors in my thoughts are.
1. The truck is to heavy.
2. It only gets 5 mpg.
3 it has 780k on it and to rebuild the motor isn't worth it because it's a prostar and a 2009.
I think my money is better spent to get a newer truck and alleviate thoes 3 problems.
And make me more available and productive.
If I go from 5mpg to 7 MOG that would be about $3500 in fuel savings.

 

Are they that bad that I will hate it?

 

And if they are what else would you susjest?

 

I totally understand it
I wouldn't rebuild the engine until it was necessary but it does have 780+ miles on it and I don't want to be in the situation whare somthing went south and it's motor time. That's only one of the reasons. The other 2 are just as important. I do relize I could buy someone's lemon. I am willing to deal with that when that problem arrives.
I'm hoping to get into a 2019/ 2020 with some remaining warranty.