Gas can mount on top of spare tire on back wall of trailer

The only thing I have seen is a fold down shelf in the back on certain campers....I forgot the make and model but that would be a mod that goes beyond what you have done and perhaps what you need.

The campers in question if I remember correctly do not have a pass through cargo and that is where the cargo is strapped down and carried.

Here is a link that you can see what I mean.