If it is just a comfort thing, my particular setup has a 125" and has just about all comforts anyone would want including a bathroom and shower ect. And, I can still haul a little over 10,000lbs. Again, the carrier is somewhat of a issue as we never haul loads that are that heavy. Our typical load is 2 to 4000lbs. RichM hit on some critical operating costs relating to tires, milage, insurance , additional loan interest,ect. Also on the comfort side, having a second axle running on the ground will give you a much rougher ride. Simply put, it is another axle that has to absorb every pothole and bump that you run over. Every turn will scrub your back tires, thus wearing them out much faster. Not to mention as RichM did, you are replacing 8 much earlier instead of 4. You are going to spend about 300 per tire unless you run retreads, and they are about 200 to 250. I have owned tractors in the past and I would put my t300 up against any class 8 tractor that is not a single axle. Remember, I said a single axle. A class 8 with a single axle will be a little smoother, but not by much.
Prior to them becoming FedEx we had a truck with Roberts. And when we did a cost comparison on lift gates and reefers we could not justify the cost on a new unit. The milage rate can't cover the additional costs. That doesn't include higher fuel for reefer, aditional insurance ect. The only way to recover these costs is to go to a used truck that has these items on it. Kinda like letting someone else take the financial hit.
Seems like it gets ignored until it is too late, is that if you spend 100,000 vs. 50,000 for a chassis you at least will spend more than double the interest costs. If you go beyond the five years, it gets even worse. On a 10% loan those numbers are scary. And if you really want to shock yourself, wait till your warranty runs out, and you are making huge payments in addition to repairs. Alot of fellow experienced drivers have financially crashed because of it.
Be careful that truck fever doesn't get the best of you.