Don't see a problem either way, just make sure there are ventilation holes so it breathes and dries out. I wouldn't take the dealer too seriously about the bees and hornets unless you hardly ever use your trailer. In time, especially in the fall, they look for places like that, but seems to me a good shot of raid through the vent holes would dissuade them from nesting there. And it wouldn't hurt to remove the hose when you arrive home and wash it out good if you haven't done that at the trailer dump. Normal care and frequent use will prevent the pesky bugs from making the Funfinder home. I have mounted my sewer hose in a pvc pipe under the rear of the X139. Just make sure the screws you use are not long enough to go through the plywood floor and penetrate your vinyl flooring. The front under the hitch area is also a good place, maybe better but I am happy with mine and it is right next to the water dump valves and convenient.
2007 Funfinder X139
2004 GMC 1500 PU crew cab 4WD