larger vents on floor on each end of 1970 mobile home

We have a 1970 Elcona. On each end, on the floor, near the wall, is a vent about 10" x 12". Heat does not come out of them. We figure it must be some sort of ventilation. What are they for? Can they be covered up, since we can feel some cold air from them, and mice seem to appear there? Thanks.