The holding tanks in a used RV are notoriously difficult to inspect before making a purchase. Don't let the additional expense be passed on to you. The holding tanks should be professionally hydro-jetted if you're purchasing from a dealer first BEFORE YOU SIGN any paperwork.
Buying from a private seller?
Make sure the tanks have been cleaned by a professional first BEFORE YOU SIGN any paperwork. Recently purchased used RVs have incurred additional costs for the new owner as a result of the previous owner's negligence. Struvite growth can be as thick as an inch thick, in addition to calcium buildup. In order to clean the tank, you cannot use a water hose or sprayers. The tanks must be Professionally cleaned at this point to protect your investment also to insure a happy and health environment.