During a recent trip to the PNW we looked at some property on the Long Beach Penn. and found the same time limit requirements as you are talking about. I believe as someone else stated earlier that in order to include RVs in the subdivisions that they had to be considered temporary structures for Zoning and Constructions purposes. They are therefore, not subject to the UBC, NEC and all the other Insurance Underwriters safety standards. RV safety and construction is more or less self policed by RVIA and as such are not included in the Town, City, County, State and National building and safety standards. There are some exceptions especially in regards to the chassis of RVs.
Some CC&Rs are slanted toward keeping up appearances and as so many of us are aware, RVs left in place on a permanent basis for occasional use are often unused after the novelty wears off and become shabby looking very quickly.
The PNW weather including where you are looking can cause deterioration very fast if proper maintenance is not completed in a timely manner.
One of the reasons we decided not to buy a lot is that then we were tied more closely to the location and we don't really do much destination camping, we drive around a lot.
Just some of the probably reasons for time limits.