If I were thinking of doing this, and I'm not, I would look long and hard at the strength of the main beams of the trailer. Using only one axle would certainly put a lot of strain on the support beams of the trailer. Three, or even two, axles spreads the strain over a larger area. I think with only one axle the main beams will start to bow.
Also a 40' toyhauler would be extremely hard to balance since the weight changes so much depending on whether it's empty or loaded. All in all it seems to me to be a bad idea.