Actually, if this was purchased as an "incomplete" vehicle, i.e. cab and chassis, since Earthcruiser was a legit motor home manufacturer, they can legally up the GVWR to what they have legally submitted for. They had to submit to DOT for approval. It's not likely they took a stock pickup and removed the bed, added a fixed camper and then slapped a sticker on it. This is the same way ambulances are done. The ambo manufactures buy the cab and chassis as "incomplete" vehicles and then put their own manufacturer spec GVWR sticker, that is DOT approved.
I believe you are mistaken.
Unfitters do not need approval by DOT. If they are willing to take legal responsibility they can slap whatever number on there they want. Its up to them if they really did in fact work with the OEM on reengineering the vehicle and did all the extensive retesting to meet FMVSS.
So what did EC do besides a lift that made the brakes, frame, axles, steering etc. able to support over 1/2 a ton more than the max of any OEM F350 single wheel Vehicle? With EC gone who holds liability for claiming its a 13,000 lb GVWR.
FROM NETA
In a Jan. 3, 1972 interpretation to the NTEA (then known as the Truck Equipment Body Distributors Association), the National Highway Traffic Safety Administration provides the following guidance: “The information supplied to the final-stage manufacturer by the incomplete vehicle manufacturer under Part 568 is to assist the final-stage manufacturer in completing the vehicle in conformity with the standards, and certifying in conformity with Part 567. There are no requirements, however, as to how the final-stage manufacturer uses this information.
“If they wish to take on the responsibility of changing the ratings in either direction, or to disregard the conformity information, that is their right. Of course, they will be assuming legal responsibility for whatever changes they make, as indicated by the facts of the particular situation and the information contained in the incomplete vehicle document. Similarly, they have the right to make whatever physical changes they wish to the chassis, and as such assume the normal responsibilities of a manufacturer in doing so including the responsibilities for the FMVSS standards affected by the changes.