Huge opening for mice in 2012 Roadtrek SS

Picked up my Roadtrek SS from my farm to get ready for traveling and discovered mice droppings all over! I had used it two weeks before and there were none then. Pulled out my Travesak and foam bed topper from the cabinet and found he/she/they(?!) had ruined them making a nest. Spent the day cleaning and putting out sticky traps. Caught one in the water pump compartment to the right of the bathroom. There is a huge 26" long perfect mouse entryway from the outside there that I discovered. The flap to the left of the sliding door that covers some plumbing and an outside outlet does not reach the bottom and leaves about 1/2" clearing. I could see daylight from inside the pump compartment! Has anyone dealt with this flaw? I know that mice can get in small holes, but this is an open invitation!
Thanks,
Dixie