I've been thinking of doing the same thing. My generator slide is pretty heavily built and when the slide is in all the weight is on the tracks so I don't think a little extra weight would be a problem.
My generator slide has a void space in front of the generator to the fiberglass front cap. I have seen several posts where people make a carrier that can hold supplies to clean the coach, hose, sponge, brush, cleaning supplies. Could possibly even mount a small tool box up there, provided it was weather tight.
I've also seen where people store a small step stool on top of the generator, but you have to have enough room an no obstructions for it to hit. I installed an auto start on my generator and I mounted the control module above the generator on the firewall and the wiring harness would probably impede my ability to do this.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi