Like Tunce, I used the existing RVQ bracket to add a shelf; maybe not as finished as his, but functional. Mine inserts onto the two arms - no parts other than 1 2*8 cut down and glued up. Note that the grill arms are available from the manufacturer, the link was posted here on JOF a few months back, try a quick search for RVQ bracket or similar terms.
ETA: there is no weight limit listed on the bracket. It is very sturdy, and I would be most worried about damaging the siding on the TT where the bottom of the bracket rests. That spot takes the most of the force.
Okay, that may have been a wild goose chase, because I couldn't find the link to the manufacturer site anymore. But I did find this link that should work. $25 for the part: