The RV park we stay at in Florida for the winter months have recently required those people who only occupy a lot for three months or less, have to remove their RV from the lot for the remaining months of the year. Originally, we were allowed to leave the RV on the lot and pay a monthly rental fee substantially less that the monthly lot rent when the RV was occupied. As a result of that requirement, the ownership made space available to store the RV for the months it wasn't occupied (for a fee). Friends who arrived down there this week have noticed that some of the rigs in the storage lot have considerable staining on the roofs, a condition that didn't occur when the RV was left on the original lot in years past. I have included a couple of pictures, showing that some have stained roofs and some don't. They are stored in an area away from overhanging trees, so I'm thinking that tree sap is not the issue. It seems odd that some are stained and some aren't. Any opinions as to the cause and how to get them cleaned would be appreciated. Thanks!!