We also have 50 gallons plus 15 in the hot water tank. We count on two showers each but get nervous trying to get a third. It would really suck to run out while you are soaped up
We also do the get wet, soap up, rinse off.
+1 on how large is the gray tank. A trick we learned is to fill a bucket while waiting for the water to get hot. Then either dump it in the black tank or outside. Fresh water can't hurt much but you may have to explain it to an interested onlooker.