Geof -
One other item to consider on the single vs. dual axle. If you will be building a 28 or smaller motorhome and will not be pulling a big transporter a single axle will ride much nicer. With a single air axle your rear rating will be 20,000 lbs and double that for the tandem. Do not even consider springs in the rear, your ride will be horrible.
I will also mention that my 28' conversion is 40' long overall with a wheel base of 336". This is lonnnggggg! I went from a 30' motorhome to 40' which is only 10', but it is a huge difference in maneuverability. All secondary road turns really need your attention and you can very easily get yourself into trouble if your not careful. I have seen quite a few 45' and even 50' conversion and I can't even image how you get those around secondary road intersections
2003 28' Show Hauler Motorhome on a 1995 FL 120
www.showhauler.com