I use the Adco cover on mine. I used it for 2 years on my TT and it did a nice job. It kept the water out, has plenty of vents, and it fit well. Also dropped down past the bottom of the trailer.
I just got my Columbus 295RL Jan 12, and got my cover a few days ago. I just put the cover on this morning and the 5th wheel cover is even easier to put on than my travel trailer. The top portion of the cover is fairly thick and I have never had any issues with anything puncturing it. The sides are well vented and made with a slightly thinner material which helps it breathe, but sheds water. The cover I got is for a 34'1" to a 37' (Part # 32856) so it should also fit no problem on your 320. The straps are easy to use and they even have weighted straps that you can use to toss the straps from one side to the other under the trailer. Even though I have only been using the adco covers for 2 years (going on 3), it has held up well to all the rain, snow, ice, and wind.
2013 Columbus 295RL
2006 F350 6.0L Crew Cab LB SRW