On my '98 Coachmen, the sticker is in the back of an overhead cabinet above the sofa in the front living area of the motorhome.
I'm a bit confused as to why you'd care to register a vehicle that is not street legal (unless the lack of plates is the only thing keeping it from being street legal). I think most states require you to assert, as part of the registration process, that the vehicle is in good repair and meets all necessary standards and so forth--in short, you in theory can't legally register a vehicle that isn't street legal. I may be mistaken, though, particularly as the laws do vary from state to state.
You could always hire a tow company to tow it to a scale to be weighed, and then either park it there while you deal with the DMV and get plates to drive it home, or else have it towed back. While not cheap, I think it's often possible to get a little better deal on a tow that is pre-arranged for a convenient time rather than needed in an emergency situation.