we have a 27' T/T and live on the Oregon coast so its damp to say the least. I stored our T/T under a carport for the winter and it started to show signs of dampness. I tried a heater, all that did was to cause the damp warm air to condense on the windows and vents and sometimes on the ceiling. we then bought a dehumidifier, an IVATION from Amazon its small and runs on 12v with a converter 110 to 12v. for storage its the best way to go and this one works excilent. just plug it in and close all windows and vents, I open all the cabinet doors and refer and your good to go. it doesn't have a drain but if you dump every 2 weeks or so its good, you could also very easily put a drain on it. our T/T is perfectly dry inside and smells nice and fresh. check it out on Amazon or Google.
Jay D.