Mark & Sherry -
Welcome to the site! A single rear axle is no problem, my conversion is exactly 40' with a 336" wheel base. I think the question you need to ask yourself is how are you going to use your conversion. If you plan to tow a racecar hauler than leave the second axle on. But if not and you like a real smooth ride than drop it. Everytime I take my conversion out on the road I am amazed at how nice it rides and handles even on rough roads. I added air ride to the front axle as well and I can honestly say it rides better than our Windstar minivan and that thing is smooth.
Good luck on your conversion and keep us posted as you move forward.
2003 28' Show Hauler Motorhome on a 1995 FL 120 www.showhauler.com