It all depends on what you want to do.
As others have said, the aircon will take a bit, mostly for the startup surge. I have a 3 ton ECU (so 36000 BTU I believe) that I easily run on a 5KW MEP802A. It sits at about half load, or a little less in aircon mode. In heat mode it's 10KW so my 5kw won't pull it.
Then it's all about whatever else you plan to run. A few LED bulbs will be negligible, incandescents will be significantly more.
You could probably toss in a couple batteries, a solar panel or two on the roof, and then use DC lighting, add an RV fan for ventilation, and not even worry about the generator.