Venting gen exhaust thru RV garage, need suggestions please.

Why not use the double wall 3" vent pipe that is used for gas hot water tank venting? It comes in various lengths, just use the silver tape to seal the joints. You can go out the side wall, or up the wall to the roof for venting. It can be portable, (just take sections apart and store when not in use), or mounted permanetly. Also, auto supply houses sell a flexible metal tubing for venting exhaust, maybe a combination of the two would work for your application.