First time seasonal.. question on tanks and slide?

We have had a seasonal site since 1993. We are basicly weekend campers. We open the slides April 1 and close them October 15. We haven't had a problem doing this. The awning is ALWAYS closed when we are not there. I also finish filling the grey and black tanks and then dump before we leave Sunday afternoon. The black tank I fill and dump twice then I close dump valve and add around 10 gallons of water to it.