Both on frame and off frame modulars are reported on the 1004. Nothing on the 1004C refers to modular homes. Never report a modular on a 1004C.
With the 1004 include comments and photos of the crawl space, any insignias, labels, tags, data plate, etc, etc in the report.
Also comments that the property will not be eligible for either Fannie Mae or Freddie Mac. They use to be for Freddie, but they have gone the same route as Fannie. All homes on a frame according to F/F are considered manufactured homes. However homes on a frame are not eligible for F/F if they are not originally constructed to the HUD building code. Modulars are not constructed to the HUD building code. They are constructed to a local site built building code, the same building code as the surrounding site built homes.
On frame modulars are eligible for FHA or USDA or private financing only.
Also explain what procedures are required in your state for the on frame modular to be taxed as real property. For example here in Arizona the manufacturer issues a title to the on frame modulars just like manufactured homes, park models, mobile homes and vehicles. That title has to be surrendered to the Motor Vehicle Department and an Affidavit of Affixture recorded in the county where the on frame modular will be installed. Off frame modulars in Arizona receive a bill of sale.
As far as comparables you might have manufactured or site built or modular homes or a mixture of all three. It all depends on what the market reaction would be to that specific on frame modular.
Explain, explain, explain some more.
In the manufactured housing section of this forum is a link to Fannie Mae that describes the insignias that are issued in each state for a modular. They are different than the HUD label. The more recent modulars also have a piece of paper some where inside the home that is similar to the HUD data plate.