If you're not fussy about cabinet dimensions and finish, just go to IKEA, Lowes or other big box store and get standard ones. Some of them can be flippin' heavy tho. as they may be made from MDF. You can sometimes find used cabinets at building recycle places or on CL.
The way RV cabinets are put together is pretty straightforward and something any handyman type can do. You'll need a table saw and maybe a chop saw though. Doors are the same as you'd find in a house but the "wood" face frames are lightweight wood sorta like balsa wood with a very thin faux wood paper-ish wrap. Making a clean cut without messing up the faux wood wrap is a challenge and you can't sand the edges or use wood filler.
I ordered some 1x2 faux wood from our dealer to match our interior cabinetry when I made a new recessed storage cabinet from scratch (see below). The wall was originally blank with a large unused space behind it. I also ordered some standard size doors from the TT manufacturer. Some plywood suppliers have a wide range of thing plywood that could be used. You might be able to get standard luan from an RV dealer.
Upper RV cabinets usually have a hollow bottom which is sometimes used for wiring, receptacles & lights which can be helpful.
There's a few ways to put the face frames together - pocket screws, brads or biscuits. The luan plywood for bottoms, sides and interiors is usually stapled together with tiny staples but you can use an air nailer with short nails. Could use some construction adhesive too. If weight isn't much of a concern, you could use any 1x2 lumber like maple, oak, etc.
The inside of RV cabinets isn't always pretty like residential ones and you'll find 1x1 screwed into wall paneling. If you plan to put heavy stuff inside, you might want to fasten to wall studs and maybe even roof joists (I did that in a previous TT). The cabinet boxes are usually assembled with 1x1 (3/4"x3/4" actual) wood that looks like pine.
One nice thing about building your own is besides a custom size, is custom features you wouldn't normally get in an RV.