I will attest to high flow intakes makin your car louder. Boy my engine volume is so high, I still don't really know what my exhaust sounds like. (which is actually a good thing cause I don't have to hear that raspy fart can of mine) Also, I have a friend that put a Weapon-R intake on his Camry and boy just the intake made that thing loud with a stock exhaust.
geez, good luck finding a new intake for your truck. I just looked on Jeg's and couldn't find a single one that fit your truck. Then again I only looked and K&N and AEM intake systems.
Almost identical mileage towing and unloaded eh? How fast is it unloaded? If not much faster than loaded I'd have to say you have a cam that is designed for heavy load towing, and probably have rear gearing in the 4.10 range. Changing the rear gears to a lower ratio and installing a new cam with a good low to midrange RPM band will work wonders for you. A wide LSA will also help keep cylinder pressure built up on the low end.
A catback exhaust that leaves the factory manifolds in place also does little more than adjust the exhaust note. The Big 3 really didn't focus on anything more than unit cost on the exhaust until the late 90s/early 00s, modern exhaust is now as effective as shorty headers. But, 80s exhaust will see a drastic improvement in power and economy with the installation of a nice set of long tube ceramic coated headers. Long tube headers emphasize the low RPM torque range, which will also help keep your towing ability.
Next up on the bandstand is electric fans. I saw a 2MPG increase with electrics alone on my 96 Tahoe. And, GM is now using electric fans on their trucks with no loss in towing ability. You want at least 5000CFM capable fans, and you want them in a shroud that will set the fans at least 2 inches off the radiator and not allow the fans to draw air anywhere but from the radiator. If you have metalworking ability I suggest building a shroud and using two Permacool 14 inch (or 16 inch if your radiator is large enough) 2950CFM fans with a DC Control fan controller.
These changes will affect reduce top end power and will lower your max towing ability, but should return a much higher fuel economy when unloaded. The fans will bring back some of the top end power. And, these changes can be done in a weekend.