I wouldn't think that the 5K unit would be too large for the 13'. Although my trailer is technically 3 feet longer, the front three feet is in the enclosed bathroom, so I think the cubic feet isn't much more than yours. I know that doesn't help much ... other than to say that I don't think it's the size of the unit. Where do you have it mounted? Does it have plenty of ventilation?
Even if you had a bathroom in your 13', it still shouldn't be doing that, I don't think.