The hole is unprotected. Urine gets into the rivets, runs under the coating, and drops out through the holes. Examine how walls are attached, and the quality of the connection. Inspect welds to see if the connection is sound or cracked. Examine inside doors or windows for any unpainted steel where the manufacturer might have skimped.
Structural rust can start in such locations.
Inspect the roof, both from the ground and at roof level. You can see stress cracks and splits. Sunlight deteriorates the coating, or the driver could hit a tree branch. Take A Test Drive Bring a towing vehicle whose hitch and electrical connection match the trailer. Check the coupler for tightness and security and that all lights work.