I showed this little item to a couple of people at the Rally; they both expressed some interest in it so I thought I would post it in case anyone else might be interested.
The item is a small meter to measure the number of gallons that flow through a garden hose. It could be used to measure the amount of water being loaded into the fresh-water tank or it could be used, as I use it, to measure the amount of water being put into the black tank during a rinse / flush operation. It is not a precision instrument by any means, so I wouldn't try to put 37.5 gallons into a 38-gallon tank, but I would be comfortable putting 25 gallons or so into an empty tank without overflowing it. [The only downside, of course, is that you can't go off and forget about it--you need to monitor it during the operation.]
It may well be available from other sources, but I got it from Lee Valley Tools at
http://www.leevalley.com/garden/page...t=2,2280,33160.