how can a plugged muffer cause poor MPG

It could only be 85% plugged up if it has fibre glass packing or similar, but normally a truck muffler is not constructed in this manner, so the dealership maybe telling lies. FYI a restricted exhaust system will cause the engine to work harder to expel the exhaust gas and therefore more fuel will be used.

 

All I can think of is Eddie Murphy sticking a banana in the tailpipe in Beverly Hills Cop!!!

 

Many of the cheap mufflers are a triple chamber design, and when a baffle breaks it will work itself up and plug off that muffler. It is not that uncommon, I have seen it in quite a few units, at the shop, and a few on the road, you would see exhaust coming from both mufflers after he was stopped at a light then when he got into a higher gear it only came from one.
We were going to the boarder so I stopped and told him.
Mufflers for Cats, bridge engines that needed catalitic converters to lower the pollution levels, it is a common problem.

 

And some are just plain confused......

 

Just a thought.... Your truck isn't equipped with a DPF is it? I may be totally off-base here, but with more and more being so-equipped, perhaps it's possible they said muffler but meant DPF?
Just a thought.
If there is no catalyst or DPF and you really have a non-baffled design with a straight shot through I bet they are blowing smoke up your backside.

If I'm completely offbase, sorry.