Let me start off with a Disclaimer all of the suggestions below are what other RVers and Campers have claimed to do to quiet down their generators. I have not personally tested any of these solutions to see if they actually work. Some of these solutions could damage your generator as well as void the warranty on a new generator.
2. Some people attach a hose to the exhaust of the generator and put the other end of the hose in a 5 gallon bucket of water to dampen the exhaust sound. They suggest that you put a small hole at the high point of hose so that your generator does not suck any water into it.
3. You mentioned building an enclosure for the generator. I ran across this video on how to build a doghouse for your generator on the cheap. Again I cannot confirm how well this works.
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