Adding a valve on the toilet?

The toilet in my camper uses a lot of water every time we flush. We typically camp at state parks without a sewer connection, and the black tank gets full quickly because of the large amount of water from flushing. Has anyone ever put some sort of valve on the supply line going to the toilet? The idea is to turn down the amount of water going into the toilet so that the black tank doesn't fill so quickly. If so, I need a recommendation on what type of valve. Thanks.