The total amount of rear overhang is dependant on a number of factors. How much ground clearance do you anticipate you will need (height of your storage bays), desired turning radius, gross weight capacity (& ft to rear distribution) of the conversion, towing capacity, more room for storage bays, and probably some other things I haven't thought of.
I wanted be able to pull an enclosed car hauler so decided to stay with the twin screw and 12,000 pound rear axles. The advantages are {obviously} a higher gvw, more towing capacity, and also considerably more stability. A side benifit (IMO) is safety, as a rear blow out would be a non issue. Disadvantage: maybe slightly reduced fuel mileage, and maybe a little more maintainance, although it would be rare to have axle or driveline problems since I'm not hauling 90,000 day in & day out.
A single 12,000 rear axle on a 30' motorhome is going to be marginal if you are going to tow any kind of enclosed trailer. You might also consider a 16,000 ft axle on a 30' rig as the weight distribution is going to be shifted forward compared to semi tractor configuration.
I also set the axles toward the back of the motorhome body, the rear axle is aprox 7' from the rear. This gives me better clearance going up (my) steep driveway - no concerns about draging the rear skirts, and excellent towing stability. The turning radius "ain't" real good, but I guess I can live with that. I also beveled the bottom of the rear skirts aprox 6" to enhance "abrupt hill" clearance.
Hope this helps.
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