Right after purchasing my XT276, I 'mouse proofed' it. That is sealing every opening thru the floor with either 0000 steel wool and/or that foam insulation that comes in a can......available at Lowes, Menards, Home Depot, etc.
As added protection, in inconspcious locations like under the tub/shower, back of cabinets, etc., I've placed cotton balls with several drops of Balsam Fir Needle oil which is a natural mouse repellent.
Or you can use Fresh Cab available at Farm Supplies stores or here: http://www.amazon.com/FRESH-CAB-BOTA.../dp/B008RG35HE
Fresh Cab uses Balsam Fir Needle oil saturated ground up corn cobs. The 100% pure Balsam Fir Needle oil is available from Amazon ~ brand NOW foods.
Additionally: Now foods also has 100% pure Cedarwood oil. A few drops of it on a cotton ball scattered around works great to repel insects.
I use the Cedarwood oil on cotton balls thru the summer 'use' season and before storing for the winter, use the Balsam Fir Needle oil to keep mice out.
Also have acouple bags of Fresh Cab and 'renew' their effectiveness every fall by adding a half dozen drops of the Balsam Fir Needle oil to them.
NEVER had a mouse in my RV summer or after winter storage and no insect invasions during the summer use months.