ok that refreshes some things now.... If you are not going to do any serious off roading, then mounting a fifth wheel RV onto the bed of the Fuso would probably be ok, though I have no experience doing this. I would assume that the suspension of the Fuso would act like the suspension of the stock trailer, but the mounting may be tricky between the frame, the flat bed and the removable RV.
Converting a factory crew cab FE with 165" wheelbase to 4wd and stretching the frame should entail the following:
Purchase and mounting of solid axle - A Dana 60 should be fine and could range from $2000 - $8000, depending on new/used, builder, gears, lockers, ratings, suspension brackets, etc. I would suggest a simple leaf spring suspension to start, but there are other options available such as links and coils.
Sliding a camper onto a flat bed is a good idea, but your math is off a bit. Yes, the GVW is 14,500 pounds, but that is total vehicle weight. You also have axle weights and tire weight and handling to consider. A 6500-7000 pound camper on the back of a FE might be a bit much since GAWR for the rear is only 9900 pounds. Some of the weight of the front part of the RV will transfer to the front axle but GAWR on a steering Dana 60 is only going to be 5500-6000 pounds. While the total could be 16k max, you really have to look at distribution and handling so you dont end up with the fat kid on a marshmellow feeling going down the road.
Ride can be tuned using helper air bags, over load springs and properly configured leaf packs. However, like a one ton truck, a leaf pack to carry a 7000 pound camper will ride like crap when empty or towing a 3000 pound boat with only a few hundred pounds of tongue weight. Depending on things like gear in the cab, people in the cab, water and food and gear in the RV, you could easily be over weight in one or more areas (GVW, axle, tire, etc)
You could upgrade the rear axle to something like a Dana 80 or AAM1150, which are rated at 11500 pounds and their bolt patterns would match the Dana 60 front. You would have more choices for gears, lockers, etc. A D80 from a Dodge truck could be picked up used for under $1000 with an AAM being slightly more. Both are leaf sprung. A new Dana 80 from Dynatrac might be 4-5 times that amount.
Stock size wheels and tires will let you keep your gearing higher and your drivetrain live an easier life, however you are still pushing close to max GVW with a 175hp/400 foot pound engine so you are not going anywhere fast or with great fuel economy. You will have to increase tire size slightly as the stockers are only rated at 2700 each in the front and 2500 each in the rear and you dont want to be at the max rating of your tires ever.
Fuel economy is better with higher gearing as that generally means lower RPMs, but there is a careful balance too when pulling heavy loads, though you did say you would be empty most of the time, you dont want to be miserable when the camper is loaded. You will go slow up hills regardless of your gear choice with this weight and a stock motor, lol.
Someone posted an FG transfer case for $900 recently, but not sure how that would mate with the 2wd FE transmission. A divorced transfer case like an Atlas or JB205 can be had from $2000 and up. This would not include things like lengthening drive shafts, cross members, shift cables or linkage, etc.
Will have to talk to Matt about labour and materials for the frame stretch and I have no idea on things like leaf packs and wheels/tires, but hopefully that answered some questions for you. As you can see there is quite a bit involved here even though making an FE into a 4wd is the simpler of the two paths IMO. If you stretch an FG, your axles and wheels/tires are still going to be overloaded with your planned RV addition so you would have that to do in addition to a cab swap and frame stretch.
Going back to your post from yesterday, I would say that purchasing a crew cab from Busbees, swapping that cab onto a stock FG that you purchased and stretching the FG frame to your desired wheelbase should be under 13 grand. Im not sure what else, if anything, was included in their bid, so I would need to know more specifics to give you more specifics, lol.
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