Class C motorhomes are conventionally seen as mid-range recreational vehicles. The stylistic feature that universally identifies a Class C motorhome is its over-the-cab body extension. Colloquially called a "granny's attic," this space is usually given over to sleeping accommodations or storage. To build your own Class C motorhome you would almost certainly have to start with a project vehicle that already features a granny's attic, because the logistics of constructing and waterproofing an aftermarket addition would be astronomical.
Source a suitable project vehicle. Aside from purpose-built Class C motorhomes, over-the-cab construction is most commonly seen in large rental cargo trucks, where the granny's attic is used by self-movers to accommodate lightweight, fragile goods. When retired from rental duty these vehicles are typically sold at auction or by the rental companies. Do research to determine the price of a good used truck, and learn how to identify an overly abused or worn out unit.
Decide how you will access the living area you intend to build in the cargo bay. Most over-the-cab trucks do not have a door or opening from the cab into the cargo bay, nor do they have a personnel door from the curb -- or passenger -- side. It is unlikely that you will have satisfactory camping, recreational or residency experiences if you have to slide open the up-and-over roller door that typically makes up the entire rear wall. Moving trucks usually do have roof vents, but if yours does not, at least two will have to be fitted; one above the galley area and one above the bathroom.
Determine how you will divide the space in the cargo area of the truck. The basic provisions a Class C motorhome must offer are shelter from rain, hot days and cool nights; theft deterrence; electrical and plumbing systems; a cooking area; a bathroom and sleeping accommodations. Beyond these basics, design the conversion to suit your own specific requirements for access, entertainment and storage. Visits to RV shows and dealerships will help you assess what you can realistically expect from your project.
Use graph paper to map out your conversion project, first transferring the actual measurements of the truck and then sketching in the doors and windows you plan to build, the dividing walls and the equipment, appliances and storage closets. Photocopy the plans and use different copies to draw in the electrical system and the plumbing routes.
Insulate the project vehicle. Moving trucks have strong, rigid frames but thin, single-skinned bodywork that lends itself to gluing rigid panels of thermal insulation between the lateral rails and the support hoops. Install the wiring circuits and plumbing and sewer systems, then line the interior with lightweight panelling. Install collapsible bunk beds or a full-size household mattress in the granny's attic. Repurpose kitchen units from home improvement warehouses to suit the size constraints of the moving van, and install lightweight closets and cabinets. Purpose-designed bathroom facilities can be sourced at salvage yards that stock accident damaged travel trailers and RVs.
The cost of building your own Class C motorhome from a moving truck is unlikely to be recovered when you sell the vehicle.
The repurposing of a vehicle is governed by formal change of use regulations in many states. Do not use your new camper on the highway without first having the vehicle weighed and inspected.