Wall mount AC Leaking

Most modern window ac's are drain is slanted to drain to the condenser coil to be slinged into to it and evaporate while adding efficiency to the window unit. That means if you place it on a level surface it would drain as designed. Most problems occur when crud, leaves, bugs, or other junk trap in the drain channel. The other issue is the condenser coil getting dirty, not allowing the condensate to penetrate.

I had a class B motorhome that used a window AC and it laid in the compartment flat and never had any issues with the drain. Of course as a HVAC tech I routinely did my annual service.