You'll NEVER be able to seal it up, mouse-proof. If it's cold out, they'll get it. Best thing is to kill them once they're inside. I've been waging all-out war with the mice this winter, and the Rat Zapper is my primary weapon:
Rat Zapper Classic RZC001 Humane Pest Trap No Poison Killer | eBay
I've moved the trap between the XLR, the wife's Jeep, and my house garage, so they don't get too used to it being in one place. The results, since Thanksgiving week:
TW: 36
Mice: 1
Only 2 have been in the XLR (and that includes the little ba$t@rd that got into my furnace and destroyed the fan on our last trip, that's where their 1 point came from.). For some reason, they love the wife's Jeep, that's where I've bagged most of them--they build nests in the glove compartment, in the heating ducts, they truly are little $h1t$.
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2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)27 ('17) 20 (‘18)21 (‘19)23
2019 Honda CRV (camping support vehicle)
2014 Harley Davidson FLHX (XLR cargo)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)