DRTDEVL -- My first (and invariable) caution is "bigger isn't always better." Also, remember you're driving a big truck so don't expect sports car acceleration. (Driving style has the single biggest effect on fuel economy ... moderate acceleration and minimal braking will improve economy.)
Exhaust back pressure is important so there's a fine line between too small and too big. 3 inch is not too big of a jump from 2.25 inch, especially for a heavily laden truck. (The same OEM exhaust is used across the range of 1/2- to 1-ton and vehicle use.)
OEM exhaust has to span the range from lightly to heavily laden, while keeping economy in mind. (Fuel and manufacturing economy.) Back in the mid-70's the general population started driving light trucks without frequent loads so manufacturers were able to go to smaller exhaust ... mostly to cut manufacturing cost.
Dual or single exhaust? If dual, X- or H-pipe crossover? You've already redone your exhaust so it's not worth revising. For others considering an exhaust change, dual (with X- or H-pipe crossover) is always better, if you can manage the additional cost of more pipe and more work.
EFI will be a significant improvement (all around) over fixed-jet carburetors. In addition to better fuel economy, it will solve the too rich/too lean issue.
Multi-port injection is better than single-port but costs significantly more. The biggest improvement is from fixed-jet to EFI and the improvement of multi-port versus single-port is minimal in comparison. (If you can afford it, there are multi-port injection intake manifolds available ... Edelbrock makes one for LA small blocks as well as other companies.)
Next step is to advance your timing a few degrees. A more complete burn leads to better use of available fuel and improved economy.
Your driving style will determine how much to advance the timing. You want the timing to be just short of pinging/knocking. If you're a 'lead foot' driver, you're probably better leaving the timing on stock specifications. If you are (or can change to) a less throttle type of person, you can get 1-4 mpg improvement by advancing timing. (Again, stock specifications cover the range from light- to heavy-throttle use.)
Changing from mechanical to electric fuel pump (in or near the tank) will solve other problems but won't affect fuel economy. (Unless you have other problems, such as leaky, old, too small, or substandard fuel lines.)
Be careful to not go overboard on the electric fuel pump. High volume/low pressure is best. Too much pressure will lead to other problems, including flooding or fuel system leaks. Your EFI instructions should specify appropriate fuel pressure (at the metering unit). If not, contact the manufacturer.
Think about fuel pressure as like taking a drink from a water hose. Reasonably easy from a low pressure (30-60 psi) garden hose. Difficult to impossible from a high pressure water line. (Along the same lines, that why there's regulators on breathing apparatus.)
Do NOT bypass the ballast resistor! At a minimum, you'll wind up wearing out plugs, cap, and rotor much, much quicker. You could very well burn up ignition system components, such as coil and ignition module, due to constant higher voltage.
Ignition system components (obviously) can withstand brief periods of higher voltage. Constant higher voltage does not allow the components recovery (mostly heat dissipation) time.
Electronic ignition is especially sensitive to extended high voltage. Imagine replacing your computer's standard 6V power supply with a 12V unit or running a 120V light bulb on 180V.)
Advancing the timing will do what you're trying to do with hotter spark. (More complete fuel burn.) Hotter spark in an otherwise fully functional ignition system won't change fuel economy set to stock timing.
There's plenty of literature on the internet about how to 'read' spark plugs. Read and follow that before deciding you need a hotter spark or hotter plugs.
In general, black sooty deposits indicate a too-cold plug. Plugs that wear out quickly (assuming adequate quality control) indicates high ignition system voltage or the wrong plug. Difficulty starting and poor fuel economy are two indicators of plugs that are too hot. (Yes, you're going to need to drive your motorhome some to diagnose the ignition system via plugs.)
I strongly recommend staying with a stock ignition coil. I've never had to change coils unless one has gone bad. On the other hand, I have managed to tweak better fuel economy without changing coil, mostly by changing driving style and advancing timing. (4-6 mpg improvement is typical for me.)
The aftermarket coil you mentioned is intended for sports cars and high performance (race car) engines. For most people, it's a placebo -- serving primarily as a mildly expensive ego boost. (The latter includes your motorhome with a stock LA small block.)
