We have two of them and they seem to do a good job. I also use one of the big Damp-Rid buckets as well. We have had no problem with dampness or mildew in our travel trailer. I just bring them to the house and plug them in for a few hours to renew them.
You'll only need one of these EVA DRY for your TT. I have a 21' hybrid so were looking at 28' in length. It just stays in the trailer 24/7 and we just use it at night when camping but in storage it's on 24/7. In August it will be in use for 2 years, bought it in Amazon for a little less than $100.
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