I am currently involved right now doing this to a 14 x 66 mobile home myself. Let me share my background with you first. I hold an unrestricted construction supervisor license, I have my HVAC universal license, I have my plumbing certificate along with my electrical certificate. I am going about doing my fully legal and recommend anyone doing building work to do the same to prevent problems down the road. The first step is to contact your local building dept. With all mobile homes, their jurisdiction falls under HUD which is handled at the state level. The building dept does not have complete jurisdiction involving mobile homes, the state and govt does. I had to get an engineer because theses type homes are engineered in a factory.
The first step is remove all your skirting so your engineer can look at the underside, your beam and pad supports, your tie downs and anything else underneath. If your underside is gone it is advisable to rethink what you would like to do. Mine is in good shape, the supports are adequate, the tie downs in good order and my framing in good shape. I have plywood floors, not particle board. If i had particle board I would use a layer of plywood and lay over my floors.
I removed all the inside walls to expose the bays. One by one i removed the wall studs which were 2x3 with 1x purlins running horizontal that the metal siding was attached to. I replaced them with full dimensional 2x4 studs, headers over all the window and door openings and any new opening i wanted to add. Replace your electrical that may be bad, I am replacing all mine completely, new plastic boxes, new switches, new outlets and pannel. I placed the wires in the middle of the walls with a minimum of 1 1/4 inches from any edge as code calls for. I used 3 1/2 kraft faced insulation r-13 stapled in each bay. You need to do your walls first before removing the roof. I sheetrocked the bottom half of my walls for some integrity, screwing it every 6 inches and leaving the taping for last. Once you complete doing this to your outside walls you are now ready for your roof and exterior siding. I have 1/2 cathedral ceilings, roof trusses and metal roofing. You need to support your walls with diagonal braces to hold it from distorting. The next thing i did was remove the metal siding and place placed sheathing on the lower 1/2. leave the top half because you will be adding a top plate before you finish the side sheathing. I am removing my roof, adding a second top plate on the walls, putting in a ridge and adding all new 2x10 rafters for insulating purposes. After installing the rafters as you remove the trusses I covered mine with 7/16 sheathing, 15 pound felt and asphalt shingles. Wrap the siding with tyvek before applying your finish which i am using vinyl siding. On the inside you use 1/2 lightweight sheet rock, do the ceilings then the top of the walls. You are ready for taping and joint compound. I added 1/4 luan to my floors i am using linoleum, roll not tiles. Install your cabinets and fixtures so you have a kitchen. I am now ready to remodel room by room inside which is where i am right now. I worked on mine full time, it has taken me 2 months to do where i got. I will button up the skirt using pressure treated lumber, 1 inch foam insulation and vinyl before winter.