2005 Nissan Frontier Severe drop in mileage after fuel line

Hello

I have a relatively new truck and had a machien gun sound happening in the truck so I would go vrooom taptaptap shift...vroooom taptaptap shift...

ok you get the point lol..so the mechaninc at the Nissan Garage said I may be having some predetionation so I should have a fuel line cleaning. I asked the cost, and when they woke me I decided against it at the dealership.

I go to Walmart and they have a fuel system cleaning using Gumout products for 22 bucks, BINGO lets roll I say...Well that was three days before Thanksgiving. I was getting 16 miles per gallon in the city and 21 on the hwy before the service. After the service and ever since I am at 11mpg in the city and 16 on the hwy..basically a 1/3 reduction in mileage. It is killing me with gas going up, that I cannot get as good of mileage as I did before..

I am stumped. I have no loose vacuum lines, I changed the air filter, the garage said they see nothing, there are no problem codes showing up (thanks to autozone) I am at a loss and ready to pull my hair out.

Can anyone help or have you heard of this before?