If your toiled is directly above the black tank, as most are then you should be able to see a pipe from the bottom of the toilet into the top of the tank. In most RVs that tube actually extends into the tank by several inches and so the level is actually into the bottom of that tube if the tank is full. It will also cause a burst of air up against the sitting user when you flush if still sitting down once the tank lever reaches the bottom of that tube so you will know. If you can see the tank under your RV it should give you a feel for how far a distance there is between the toilet and the bottom of the tank. You can probably also see the bottom of the tank when it is empty.I can't tell where the top of the tank is, i.e. how close to full.
While I very much doubt that would work that way, it is possible but I'd not open the gray until the black was empty to avoid any backup in the wrong direction. If you add a valve on the end of the sewer connection like this one, then you could close it and open the gray dump valve with it closed and the black remaining open to do somewhat of your idea. Part of the answer is in how far above the discharge the tanks as located as anything from the gray would have to fill the entire plumbing before it backed into the black tank. There are also several add on , but having used one I don't find them very effective and doubt that they would get your level gages working.If there's 20 gallons in the gray, it will rush into the black tank in seconds until the levels equilibrate.