Check for a clogged cat. exhaust shops can tell you if its clogged by temp or drilling. Also, get the truck nice and warm, look underneath at the cat and see if it is glowing RED. if it is, replace the cat, that should fix your problem. cost is about 100 bucks.
Check compression. possible valve seals or rings. check with compression tester or leak down tester.
Both would cause bad gas mileage and sluggish conditions.