Hey Carl, welcome to the forums!
I may be wrong (others will chime in), but I thought the '71 models a had all 3 tanks, at least a black & fresh water... So, the catch is if you don't have a grey water tank, either always be hooked up to a full hookup location where your grey water can drain continuously or have one of those tote along containers to drain into (since there is no onboard tank). That's a drag because grey water is useful to clean out the drain/hose after dumping your black tank...
One way to find out if you have a grey water tank is to make sure all releases are closed (they're usually right next to each other), then turn on the sink and see if the drain backs up (have you checked to see if there's any levers on the side, rear on in the bumper, make sure your sewer is hooked up before pulling any of them out..?).
If you find any levers, and pull them (while hooked up), and let them drain completely. Then close them and turn the water on in the sink. If it backs up fairly quickly, you probably don't have a grey water tank.
It's a good idea to have some water in the black tank after draining so any waste doesn't adhere to the bottom of the tank. The black water tank is filled by the toilet - you can hold the flush pedal down to add water to the tank.
It's a good idea to leave the grey water valve open during normal use and keep the black water tank closed until full. The day you're going to empty the black tank, I'd close the grey water drain, take showers, wash dishes, etc. so as to have a flush available after dumping the black water tank...
Whatever you do, don't watch Robin Williams RV movie for guidance...