Get a measured container, and fill it up. Pour it into the toilet and see how high it goes. Same with the grey tank or tanks. Use the full hook up to fill your tanks with ten gallons each, and then dump, first black, then gray.
Now is the time to test it and see how it operates. If all goes well and each flushes with no leaks, you will feel a lot more secure when it's really full and needs dumping.
You can buy a clear elbow for your flush valve. I use one on my rig so I can see what's coming out. When you think it's clean, it's good to know it's clean.