My conversion has only one rear axle.
Having two gives one the possibility of loosing traction on rough terrain when the undriven axle lifts the driven wheel off the ground and then you have to lock in the other drive.
I doubt that I will ever get near the gross weight of the rear axle. With a gross of 12,000 lbs available on the front axle, the empty truck front axle weighs in at 9,420 lbs with the fuel tanks half full (125g). The rear axle can gross at 20,000 lbs and as an empty box it weighed 8,200 lbs. So a gross vehicle weight of 32,000 lbs will give me 14,380 lbs of conversion with no more than 2,580 lbs added to the front but I can add 11,800 lbs to the rear. This means I that anything I add such as generators etc should have a 4 to 1 bias to the rear.
Another valid point is that a second (tandem) rear axle gives you 4 more tires and another set of brakes and suspension to upkeep too.
If you are building your own conversion, like I am, you will find that axles sure do get in the way of black and grey water tanks. Probably why you also find some strange looking conversions with huge rear overhangs, possibly due to the converter not wishing to extend the frame on the semi-truck they decided to use.
You will need a wheel base to the rear axle of at least 265" using a day cab and a 24ft body. Now if you are only building a longer motorhome with no intention of pulling anything long, then you will need to have a much longer wheelbase and then maybe another axle, but I would look into using a tag axle with only 2 tires and a steerable one if possible as they turn tighter and with a lot less scrubbing of the tires.
Don't forget that most States have a max length of 65ft for trucks and trailers, very few allow longer. Hence I sold my 40ft MCI bus and went for the Freightliner. It ends up around the 35ft mark and the 26ft tag trailer is a shade under 30ft including the tongue.
Hope this helps. Email me if you need more info.
Peter.
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