You have two valves in the cabinet with the TV cable connections, outside water flush connector and perhaps the water purification filter. One is the black water valve and the other the bathroom grey water valve. The valve behind the round cap is for the kitchen. Be careful with the labels on the valves in the cabinet. Ours were reversed.
As for what to do with them, it depends on whether you are dry and wet camping; if you are dry camping you will probably empty the tanks at a dump station at the end of the trip. I clean out my black valve first and then the two greys starting with the grey in the cabinet. That gives me some grey water to wash away some of the black material in the pipe. Now take your utility water hose (not your fresh water hose) and attach it to the outlet in the cabinet and run water full open for about 10 to 15 minutes. This will clear you tanks of any material left behind. (not the kitchen tank). You can periodically run water down the sink to clean out the kitchen holding tank. If you are wet camping, you can hook up the drain hose when you arrive. I do not open the values at this time, but some people do. You have gauges inside the FW showing the amount of water in each of the tanks. As they fill up you can drain them individually or all together or as needed.
Be sure and use chemicals as prescribed for the FW.
You should have an owners manual to help with this process. Get a good pair of gloves and use them whenever connecting or dumping. You should have an outside shower. Use it to wash immediately after connecting or dumping. I have soap in the dispenser in the cabinet and a bar of soap and Handy wipes. Get bags like garment bags and store hoses in them as they will have bacteria.
Enjoy your new home away from home.