OK, my head is starting to hurt.
Bobz, I may have to stand corrected on the single vs. twin screw, at least partially. While it may be technically possible to scale 80,000 with a single screw tractor and a triple axle trailer, I doubt it's gonna happen in the real world. Here is a link to the dot "bridge rule" that sets max federal weights for the highway:
Bridge Formula Weights- FHWA Freight Management and Operations
A single axle can be max 20,000#, tandems can be 34,000#, and triple axles can be 42,000#. So if you have a triple axle trailer maxed out at 42000 and perfectly balanced so your drive axle hits 20000, that leaves you 18000 you have to get onto the front axle somehow to add up to 80000. In the real world you'll never see a combination unit anywhere close to 80,000# without a twin screw truck.
Anyway, like I said my head is starting to hurt. So it seems that even though some of your trucks may have an axle rating of over 20,000#, you are still limited to 20,000# max per axle by federal weight rules. So you can spec out as heavy an axle as you want, but you can still only be at 20,000 per axle on the highway.
I know, I know, "but officer, it's an RV" not a truck. If you're toting that racecar trailer more and more states are cracking down on those of us that have been skating, they need money and the theory is if you are racing, you are racing for a payout and it is therefore a business and your rig is commercial regardless of what the license plate says, and they will enforce full dot regs just like a semi. So it's just a matter of time until we are crossing scales, unless you are truly just an RV. But let's face it, almost everyone with a truck conversion vs. a traditional RV has it to have the ability to haul a heavy trailer and/or skate on the dot stuff. Check out some of the racer horror stories on the old hippie forum.