The T-139 is so small that it's really hard to find anything small enough for it. However, I did look at National RV Covers after Twinster suggested it, and they actually do make one that would fit! But it's almost $400, nearly double a more standard, larger size. It's rated for 5 years, so that's still not a lot per year, if I look at it that way. hmmm
Thanks for the idea.