Very few stores have door blanks for Motor homes Some of the places I've found them (Tri-mark lock on my RV)
Selected RV stores may have them .. IF they make keys
A locksmith on Sahara in Los Vegas about 2 miles west of the strip (Only locksmitin in the 1-7 mile range so if you are there)
A chain of hardwar stores "Do It".
Some major locksmith places like Fred's in Detroit.. IF it's been made, odds are Fred's has the blank.
These are to be taken as "examples" of places you might find keys... No guarantees.
You can also order new keys from Tri-Mark.. EXPENSIVE