Wow,
With a 20,000 pound rear axle, you can tow a very heavy load, with single rear axle. I just wonder what you plan on towing that would require dual rear axles to tow it.
If I where to get a Volvo, I proably would change it to super singles, and go with single rear wheels. But I will not be towing something with over a 4,000 pound pin weight, so the axle capacity with single tires only would need to be about 14,000 pounds to meet my needs. 7,000 pound rated tires are fairly easy to come by, and are not as expensive as buying 4 of the original size, and needing to replace them every 5-7 years.
Good luck on your project!
I don't think you should have any problems towing a RV with a tractor registered as a "Farm Vehicle". Some states you would need to pull into commercial scales. In Oregon, they are posted "Trucks over 20,000 GVWR must stop at open scales" A RV is not required to stop, it is not a truck, however a farm vehicle would need to stop, even if towing a RV.
Fred.