On your very old toilet which probably flushes 5+ gallons every time....condensation will be a big problem. New toilets which flush as little as 1/28 gallons, that of course will be LESS of a problem, unless of course you have to flush 3 times!
You can get foam liner kits to put inside your old tank. That will help. And the old tried and true solution is to install a mixer and fill the tank with lukewarm water.