My opinion (sorta proven) is that reducing pumping losses, internal friction & rolling resistance can help with both power and MPG.
Pumping losses are those losses that occur when the engine must overcome restrictions such as the intake of air & exhaust of spent gasses. It must use it's own power [i]to make power[i/]. Hmmm??...
We all know about opening up our intake and exhaust. Sometimes this can shift the powerband up. But if done properly and thought out it can help to improve both power and MPG across the usable rev range.
Let me expand....
Ripping off the intake completely and removing all the exhaust system is not the way to go.
I have opened up my intake with a low restriction filter, did the ZooDad to help funnel in non-heated air, installed a low restriction muffler and use only synthetic oils everywhere.
This to me accomplished my "mission" to reduce pumping losses & help cut internal friction (on a budget).
Most know that my truck was wrecked a week ago and that I've been driving a 2003 V10 Excursion (rental). This rental truck has 7K on it and only averages about 11.5 city and 14 on the highway. I drive the same routes as normal and have made no changes in driving style or habit.
But this truck is all stock. Stock restrictive intake, stock tractor muffler and 5W-20 oil (semi-synth) Some will argue that it is not broken in yet but I disagree. It was broken in at 5K but that's another arguement....
MY truck will post 13.2-13.5 AVERAGE all day long! I regularly get 16 on the highway. Is it the mods? YES. I betchya that if I swapped out all the aftermarket parts for stock parts I'd lose the 1.5 - 2 MPG I gained. Plus this new V10 has the restrictive Y-pipe and mine doesn't. It was really hard for me to believe that I could get almost a 20% increase in mileage (comparitively speaking) but after 40 some odd thousand miles with the mods and MANY, MANY manual calculations...well....
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I believe that the computer will compensate for rich/lean conditions to a point. I'm not sure what mods you were referring to.
If you really want to increase mileage on the motorhome of yours play with tire pressures.