I am looking to convert a 2001 Sterling with a Cat C12 engine and a ten speed trans { large sleeper bunk} to a motorhome. Question is how much overhang over the rear axle would be appropriate. I would like to remove the first drive axle to convert to a single which would give me a wheelbase of 257" front centre of steering axle to centre of drive axle. Planning on a 30' living unit with a slideout which should make a nice size motorhome. Previously owned a Freightliner FL112 which pulled a large Golden Falcon trailer which I sold when my partner passed away. Thing is I guess I still have diesel fuel in my veins and getting itchy feet to hit the road again. Would also like a matching enclosed trailer to haul car, Air brakes of course.

...I'd go with a combination toter/trailer.....over hang will depend on finished body height......20 degree angle should be more than enough.....I solved butt drag with a 7.5' X4" roller almost all the way across the back of my trailer....so what if it drags-it's on a roller!....noisy-but no damage to trailer which is 10 inches off ground at rear....geofkaye

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.

......anyway why single?.....geofkaye

Numerous reasons to go single axle Geof. Smoother ride, Only four tires to replace instead of eight, Better traction, Fuel economy to name a few. Semis pulling a double bottom setup { 2x 28' trailers mostly have single drive axles. Tandem axles are rated for 34,ooolbs and I very much doubt a 30' motorhome will weigh anywhere near that, As for extra safety it would be quite a coincidnce for two tires on the one side to blow at the same time. The Sterling I am planning on converting is rated to gross 80,000lbs that's 12 on steering, 34 on tandem drivers and 34 on trailer tandems. Just my thoughts but any advice would be very welcome.

Was just on the phone with an old friend who has built two gorgeous units in the past and he suggested stretching the frame leaving the tandem axles on. He said 8' from the centre of the rear wheel should be good for overhang as long as there is enough height to give ground clearance as Bob and Geof suggested. 20degree angle sounds about right then Geof. That should also give good weight to the axles.
Thank you very much for the advice guys, Looks like a stretch job is a must.