Are you talking about extending off the back for add'l living space? Or just need more storage? The cost per square foot to add living space is probably more than you'd expect. You probably need some engineering expertise to ensure it's stable and resilient to whatever weight you load in it - I'm imagining some bodywork hanging out off the back with no framework for support. There's also the safety factor because the added structure becomes the point of impact in a rear end collision which is MORE engineering expertise. There's also added stress to the suspension, it'll change the handling, it may become unpredictable handling if too much weight behind the rear axle counters the weight on the steer axle. It'll counter swing more in turns. And besides all of those considerations, the federal maximum length limit for a motorcoach or RV is 45 feet so if you're starting with a 40 footer you can only legally gain 5 feet of length or 40 square feet total which seems pretty meager given the headaches and expense.
Just my two cents.