What you're calling the "air gauge", is actually a filter gauge and they are absolutely useless. You don't say what speeds you're driving, but 6.2 mpg in a Mountain Aire DP is not far off from what most get. Mid 7's would be close to normal if you're keeping the speed under 65.
You're asking for issues if you drill the air filter canister. Your current design picks up cool air from the roof line of the coach. It also filters out water through a flapper when rain water is ingested. If you drill the air filter, you'll be picking up hot air form the engine and possibly water when driving in the rain.....basically a bad idea!