Plugging what you said you did into jsbin, I can't see anything wrong.
$(document).ready(function () {
$("input.room-qty-input")
.on('focus', function () {
var hotelRoom = $(this).closest('tr');
var numbNights = hotelRoom.find('.num-nights').val();
if (isNaN(numbNights)) console.log('NaN'); else console.log('is number');
console.log(this, hotelRoom, numbNights);
});
});