It has now been over a year since I set this up and I have now checked the tank on 7 separate occasions. To check it I remove the fuel sender and send some colored paper into the tank rubbing all the walls and tank floor I can get to. As of yet every single attempt at finding "condensation" has come up BONE DRY...
Does an empty tank condensate.....?? This one doesn't....
EDIT 6/25/14: Just checked it again last night and it is still bone dry. Not even a hint of moisture. I chose to check it yesterday because humidity levels have been in the 90%+ range and night time temps have been dipping to the high 40's 47F - 49F and day time barn temps reaching 118F!!! That is approximately a 70F swing with 80-90% humidity....
EDIT 5/18/14: About two months ago I set it on some 2" thick stone pavers as some surmised that the tank needs to remain cool as it would if in the belly of a boat at ocean temp. Up here in Maine, when hauled out for the winter, there is no "ocean temp" but I placed it on the slower to change temperature stone pavers anyway.. Results, STILL BONE DRY....
EDIT 10/22/15: Topic just came up on one of the sailing forums and it prompted me to go check the tank again. Just finished a very hot summer and now we are seeing temps into the 20's at night and 60's during the day. Tank interior is still 100% bone dry. Considering the tank has been here since March 31 2013 and today is October 22, 2015 I think it is safe to say an empty sailboat fuel tank will not "magically" fill with water.
I personally prefer to leave my tank empty some prefer full. You will have to decide what works best for your boat...