Many people think a bumper is.. Well, a Bumper.. Most Travel Traliers, be they TT's or 5Ers do not have rear bumpers.. They have a sewer hose storage container.
A Bumper... Is designed to Bump with.. It can take and absorb a fairly decent impact... A hose container is very thin, metal, and a your average tree rodent (Squirrel) can hit it hard enough to dent it. The load carryig capacity is somewhere around 10-20 pounds.. (The tray that held that generator) expecting it to hold anything at all, other than the sewer hose, is asking a lot of it.