You are only 10" longer then me and if yours is the same as mine, you only have 3 tanks despite the control panel showing 2 black and 2 grey. FR seems to use a generic panel for a lot of their models.
I have the same as you, right in front of the tires in the kitchen drain for the grey water and then I have 1 black and 1 grey in the side compartment.
Although I know many don't, I keep both grey and black closed until they get close to full. I don't like the idea of food particles possibly settling in the bottom of the grey tank with the drain valve always open. I work it like the black, keep it closed and then open when it gets close to full so everything is rushed out at once.
2015 F350 6.7 Platinum
2015 Coachmen 360IBL