2001 Mustang V6 what type gas mileage should I expect?

I do about 70% highway and 30% city and I get about 24mpg. If you do city driving a lot then your mileage will not be as great. Add a CAI or a after-market muffler to help with gas mileage.

I have a CAI on the way and it has after-market true dual exhaust, I don't know what brand, I haven't gone under to look. Should I pull my spark plugs and check them? The car has 74k miles on it.

I looked at the mufflers, they say IMCO and what looks like 28904

Also should I be looking for a tuner? Could I use that to get better gas mileage?

You should always check/change the spark plugs if you don't know - plug wires wouldn't be a bad idea either. check the air filter, clean the MAF (with MAF cleaner spray, or q-tip and carb cleaner), run SeaFoam through the gas tank and upper intake (follow instructions on can).

2004 3.8 here, I get about 20-21 for 80% city, 20% highway driving with my dual exhaust, CAI, and performance 93 octane tune. So yours is about right. If I'm doing 100% highway I'm around 25-26 if I'm not leadfooting it (hahaha!)