The First Rule of Rock Climbing:
Don't let go of what you've got hold of until you've gotten hold of something else.
If you'd already built an RV and made a profit selling it, enough to support you while you build your next one, you'd have a reasonable chance at it. In any event, assess it like a business, taking account of the start-up costs, the on-going costs, what's recoverable and what isn't.
Look at what would happen if you built, on spec, an otherwise desirable airplane, only to find that the panel features the now-not-cool-last-cycle avionics, because the most wonderful one imaginable was released after you'd bought the not-so-cool thing.
Or if you snag a willing customer who ignores the hired-gun rules (look 'em up) but then has a heart attack. Or maybe everything appears to be going to plan and then you have an illness and get behind schedule and the contract lapses. Or maybee everything appears to be going to plan and then because of the rules against this, you can't get it licensed. Ever.
I'm not saying it can't work. Under certain circumstances it can. But those are very rare, awfully specific, and most unlikely to come around again.
Dave