I've heard plenty of differing opinions on covers. Call the manufacturer first. A cover MAY void the warranty as it can trap moisture and during wind can actually wear through some parts because of friction.
As far as mice, it is really just random. Before we had our 26TBSS we had a popup that we stored next to a collapsing barn at my dw's mother's farm. In 5 years we never once had mice or rats get it, and the weeds even grew up around it. Ants were a problem, but I think if your trailer is sealed up well, you should be ok, skirting won't help. I think the skirt is just for campers that aren't insulated under the belly, if you were going to try to use them in cold weather.
BUT, I'm a rookie as well, just learned this stuff from my recent experience and from my dad.