Heavy Haul truck. Highway truck.

I have a general question I wish to put forth for response in order to educate myself against what I see everyday.
I will frame my question with a statement:

"Heavy haul" truck in comparison to "Highway" truck. I noticed that dealers "market" trucks by criteria such as "Heavy Haul" and "Highway". HP, gear ratio and axle weight leads the obvious factors. What I would like to know is... I see trucks that appear to be "Highway" trucks or regular OTR trucks, hauling what I see as "Heavy Haul" loads.
My question(s):
1. Are these, what appear to be "Highway" trucks, utilizing modified gears and transmissions to fill this role or are all trucks able to fill this role, barring any considerations such as low HP or "Tall gears" or axle weight limitations?
I understand that some people/truckers will push their equipment to do things that it probably should not be doing. I am more interested in understanding how to properly choose and rig out your setup and not put money in places where it could be better utilized elsewhere.

2. When a company or individual decides to choose their direction, for instance - "hypothetical": I want to do strictly "Heavy Haul" with some occasional need to travel greater distances with a heavy load, say, across state lines or over several state lines? At what point does the equipment choice become critical with regards to longevity? In other words, I assume I will need higher HP, lower gears, increased axle weight rating and possibly a strengthened frame.

Another thing I noticed about these marketing approaches... The trucks that are presented as "Highway" trucks, they talk about fuel economy. "Heavy Haul" trucks do not have that in their marketing ploy. It is obvious that you will lose mileage for various reasons due to setup?
My question is... Won't you lose or achieve similar mileage if your truck is not typically or properly set up or modified for this type of work?
What is the trade off and how do you decide how to rig your truck?

Please keep in mind that I am a rookie. I am simply trying to understand the proper way to choose the right tool for the job. When I am able to wash the "rookie" off of myself and have several years of experience under my belt, I would like to find myself doing heavy haul work. So, I am beginning my homework now. Furthermore, I would rather hear from real truckers and not salesman regarding these differences. I am in no danger of doing this anytime soon.

 

To provide a frame of reference for my question.... I was examining the Mack website. That is how my question originated.

 

Please elaborate. You say that you took all your highway trucks and built into HH trucks?
1. What changes did you make? Transmission? Gears? Suspension?
2. How will your maintenance patterns and frequencies change?
3. Will the highway to heavy modifications affect MPG much or not much at all?
4. Can you still expect to get greater than 1 mil miles from such rigs or are they shorter life expectancy equipment?

 

1. What truck brand did you choose for this setup?
2. What engine brand did you achieve the 1 mil miles with? Did you haul heavy and normal highway with this setup?
3. What is your transmission on this rig?
4. Axle rating on this rig?
I only ask these things to understand individual setups. I am not interested in brand warfare. I know the competition is fierce among the manufacturers.

 

Nice. So the W900 was not originally built to the spec you mention? This was a modification to suit your need? Regarding your pulling weights.... 130k to 140k, is that gross combined or is that the equipment weight your hauling?
You mentioned a fourth axle. Are you referring to a trailer axle? Do you have a third axle on the truck or just tandem?