Good question Chris,
Most OTR tandem axle tractors have a 38 or 40K rear axle rating. This means that when crossing weigh scales, a commercial vehicle can legally weigh the respective amounts above on the rear axles only.
When one axle is removed from a tandem, so is 1/2 of the rear axle GVW rating. A single axle will be rated as either 18 or 20K.
When towing a fifth wheel travel trailer, the most anyone usually has on the hitch is around 4K. If you chose to carry a couple of motorcycles, tools etc., on a custom bed, you would still be hard pressed to exceed the 18K capacity of the remaining single axle. The second axle is therefore unnecessary.
Then there is the overall tractor length consideration. Removing an axle and shortening the frame allows for better maneuverability in tight spots.
However, some folks want something different. Some are towing fivers with factory tandem axles. Some drop out the front axle, lengthen the driveshaft, add a carrier bearing, install a custom bed, and haul various items in front of the fiver.
In general, a Kenworth is the only tractor that cannot be converted to single axle due to the unique suspension arrange Kenworth utilizes on most of their tractors.
To give you a better idea of a single axle conversion, below is the URL for my latest conversion. The Volvo depicted in the site is already sold, but I have already purchased another one exactly like it which has not yet been converted.
The Volvo on the site below has a 202 inch wheelbase. Concerning the other Volvo I have not yet converted, If I were to remove the front axle, and retain the rear axle in its current position, the wheelbase would be about 256 inches. This would really provide lots of room to haul whatever in front of your fiver, I would also sell it with the tandems intact as it now sits.
There are 21 large pictures and text on the site below. If you have a dial up connection, it may take up to 8 minutes to load.
http://onezman.tripod.com/igotyourrvhaulerrighthere/
onezman