My best guess is that one drain dumps the black tanks and the other the galley and grey tanks. That would be a satisfactory arrangement so might be the case. It would however, suggest that the grey and galley are effectively one large tank (levels would equalize), and the two black tanks as well. This too would be an okay design. It's a bit confusing why there would be level indicators for both the grey and galley tanks, but stranger things are found in RVs. There is just one indicator for two black tanks? That would suggest they have a common connection and will equalize and knowing the level of one tells you the level of both (if there really are two). The control panel is generic and not tailored to your rig.
I don't think most of us do much to clean or sanitize the tanks. There is no practical way to fully extract all the contents of black tanks. Even a tank sprayer (if you have this feature) or putting a hose in the toilet with water running out the dump outlet clear does not mean there aren't some solids in the corners of the tanks. On the road I dump and don't try to get everything out. I put in some enzyme stuff (link below) at our next stop and that's it. This stuff results in very little solids at dump time. I have a clear elbow in the dump hose to see what's coming out, but mostly just to know when the flow has decreased substantially and it's time to close up.
The black tanks are essentially septic tanks without a drain field. They get drained now and then but don't need anything more than some digester starter to get decay started and thereby keep odor down in case you accidentally flush with the overhead fan on.
Same with the galley and gray tanks. Empty and put some digester in. The galley tank gets food particles so can stink via the vent (sink will have a trap under it), but it has to sit quite some days for that to happen. The stuff below takes care of that. It has no smell itself, it's just an enzyme or some such that digests the food particles.
When we get home from a trip and the RV may sit for months, it's the same, put a gallon or two of water in the waste tanks and add some digester.