RV manufacturers sometimes plumb the lavatory sink to the Black Holding Tank rather than the Grey Tank. There may be several reasons why they do this. Economic (too cheap to plumb the sink across the aisle), Functional (not an adequate drain path), or Balancing (take some load off the grey tank). Regardless, if your lavatory sink is plumbed into the Black Holding Tank, you have a built-in tank rinsing system.
The idea is when you are at the dump station, simply open the faucet to the lavatory sink as you begin emptying the black tank. Then when the black tank is empty, close off the dump valve and allow the tank to fill from the sink. And when the tank is half full or there-bouts, dump the tank again.
Of course, you can do this procedure as many times as you wish, as long as you have fresh water. But of course, practice good RV manners and don't do this if there are others waiting to use the dump station. But if you have time, then you can rinse a couple of times, or until you see clear water in the inspection sleeve (and I highly recommend buying one for this purpose).
This procedure won't likely be quite as effective as the spray systems that some RV's have, but it is surely better than nothing... and it's free.