This is not a recommendation, just a datapoint.
We've had an expired quart kit of proseal in the frig at the hangar for over ten years. It has come in handy many times for various adhesive tasks. Eight years ago I built a tank for the Legal Eagle with it, last year I modified the tank on the D.VII, and this year I sealed the transmission on the
'48 Plymouth with the same quart.
It has always cured as it should.
If I was building RV tanks I would definitely start with a new quart. But I wouldn't be concerned if it expired prior to finishing the second tank. If you get all the prep work done on both tanks you can seal a pair in a month. Here is the log for my RV-6 tanks which were completed in one month:
http://home.hiwaay.net/~sbuc/journal/tanks.html