Are you sure they are mosquitos? I think most RV'ers have heard that it is a no-no to leave the black water tank valve open during use. However, many think it's OK to leave the grey water valves open when camping. I never do because I have read on another forum, Escapeesr, that this can lead to an infestation of "Sewer Fles". Apparently they reside in some in ground sewer systems and get into your tanks if something is left open. So, may be a different species of pest residing in your system.