Gas mileage has dropped drasticaly

I'm going to go ahead and blame your EGR valve, opening up when it's not the right time, possibly the sensor that makes this happen, MAF MAP etc...your trouble codes would tell you what is, if you bothered to read them.

the leak is an internal vacuum leak, causing things to malfunction.

you know if you filled up to the cap, the emissions system would have a gasoline bound canister and would take time to recover and create errors...your evarporative emissions would be buggerd for a while throwing up the CEL until it was put right. Supposed to have ALL emissions sewed-up proper, if there is a leak in which some vapor can escape, it goes into B9 control robot throwing it's claws in the air and says "warning warning". I think what we are dealing with here is one confused polution control system. Take it to the smog guy who will uncover/discover these malfuncioning emissions controls-

what year? maintenance light and check engine light are different things...if its the maintenance light then someone forgot to reset it...if it is a check engine light you could take it to autozone and have them scan for any codes for free. also what was included in the complete tune up?

The shop may be talking about the EVAP system. This emission control system recycles gasoline vapors back to the engine to be burned up rather than be released into the air (required federally since 1971, since 1970 in California). If this system malfunctions it will effect gas mileage and turn on your check engine light. I'd change the thermostat. If it's stuck open your engine won't reach operating temperature. This will also greatly affect gas mileage. HTH. -Jim

Have 2007 CR-V EX, and was getting 27-30 MPG, until I had tires replaced, MPG fell overnight to 20- 25. Complained to tire store, they said new tires, thicker tread will cause lower MPG. Still not happy with answer, 6 months later.

what kind of tires?

Most likely heavier or oversize tires with aggressive thread will lower MPG