Many openings to a home:
If a home has many openings to the outside or to the courtyard, and the openings were built for the household members to use to enter and exit into the home, then they are all obligated to have a Mezuzah placed on them. This applies even if only one of the openings is accustomed to be used. This applies even if many of the household members have moved out and thus only one opening is now used. However an opening that was not built to be used to enter and exit from, but rather for beauty, then it is exempt from needing a Mezuzah.
 Michaber 286/17-18
 Michaber 286/18
 Michaber 286/17
Accordingly, caravans that have many doors but only one is actually used, need a Mezuzah on all the doors, being that they were all made to enter and exit from.
Q. Halacha 17 and 18 seem like a repetition? What novelty is there in Halacha 18 that we did not already know from Halacha 17?
Q. Does this mean that ones an opening has been used it can no longer become exempt from Mezuzah even if they decide to no longer use it?
 Pischeiy Shearim 286/197; Implied from above Halacha