I agree, plug gap check or replace, wires that are spiral core metal, new cap and rotor. Check and clean EGR passages and operation. Check or replace MAP sensor. Check your injectors with a timing light. O2 sensor can be checked with a voltmeter.
All these checks can be researched and followed on you tube.
GM TBI cam specs are real small 382/403 lift at the valve with a 1.5 rocker ratio.
You can buy Crane 1.6 roller rockers and increase the lift to 407/429.
A stock carbed 350 cam is 390/410
A 1978 Corvette 350 cam is a 403/415.
A Vortec roller cam is 427/427.
I don't know what the Crane roller rockers cost, but it is a lot easier than a cam change.
In the bed of your truck are pockets along the sides for 2x4's to create a double layer load deck.
You can install the 2x4's and put a plywood platform on top of them for your sleeping area.
If you use a standard 4x8 sheet you can stuff and store things ahead of and behind the wheelwells and underneath your platform.
If I had one of these I would keep it stock.
If I ever went SAS, I would use a Dodge AAM front axle with coil springs, Ford or aftermarket spring buckets, radius arms and a panhard rod. This design would be similar to the old Ford Mono Beam F100/150 front ends and just like the new and improved Power Wagon setup.
Shackle flip for the rear axle.
Put some H2 wheels on it with 35" tires and use your Chevy center caps. 
I am going to tease you here, millage is a form of tax, I have no tips on getting more millage and if I did I would become part of the problem not the solution. I like to fix stuff, not mess it up with more millage, I pay enough already!