Gasoline Evaporation of 3/4 gallon per week ???????

I have a 1995 Bounder with a 75 gal. gas tank. I am loosing about 3/4 gal. of fuel every week because of what I think is evaporation. I have looked for leaks and find none. I followed the vent tube from the gas tank back to the gas cap area and it looks in tack. The vent tube is a closed loop next to the main gas fill hose. The generator does draw off of the main fuel tank but I don't see any problems with it. I also tried to seal off the gas cap very tight with saran warp to stop the evaporation and that did nothing ? Is it possible that this is normal amount of evaporation ? My climate in So. California is about 75 -85 degrees. Any help on this question would be very much appreciated.

Thanks,
Brad