A better way? to keep RTV usable.

There's 2 things I've done with RTV that seem to work pretty well:
Take a small square of plastic (piece of a bag, Visqueen, whatever you've got) and put it over the end of the tube before you screw the cap on.
Put the tube in a Ziplock bag, seal it 3/4 of the way, suck the air out, and finish sealing it. Kind of a poor man's vacuum sealer.
Both have been pretty effective.