A little help finding a leak

The family and are currently winding up a week at Apache campground in MB in our 3550. The unit is a 09 i bought two years ago and I am very ocd about maintenance being pro-active not reactive. I noticed last night that right inside the rear door there is a dark spot!!! I immediately started to freak out and searched for causes. First I looked inside the fender wells for any openings with no luck. Then i looked at the caulking and no luck there. Then I remembered that I had found my drain for the fridge behind the plastic grate and thought possibly the water had drained through the hose and ran inside the wall. I removed the cover and saw no water staining there or inside under the fridge. Now i am truly stuck and I come to my peers for help. Anything you can recommend would be appreciated.

This may be a dumb question, but do you have any pets on board? Mine have been known to make the occasional spot here and there. Trace

Yes we have a small dog with us but that bathroom is shut off from her. This llooks like water damage.