Before launching into my sermon ...
Engine (re)builders often ask, "How much horsepower can you afford?" We can rephrase that, "How much fuel economy can you afford?"
Inexpensive things first. These are aimed towards ensuring your motorhome makes effective use of the fuel it consumes and doesn't waste any.
Decades ago, an old timer told me, "Ignition first, fuel second." While he was referring to getting a stubborn engine running, it also applies to fuel economy.
Effective use of fuel requires a hot spark in the cylinders, at the right time. If you don't have that, messing with the fuel system is a waste of time.
So, new spark plugs, distributor cap, and distributor rotor, at a minimum. Make sure the spark plug gaps are correct. Most come with the correct gap but it does hurt to make sure. (Be sure to use the correct, wire-type gap gauge.)
If you have doubts about the plug wires, get a new set and make sure the wires are going to the right cylinders. Mixed up wires means at least two (25 percent) of your cylinders aren't burning the fuel they consume.
At a minimum, make sure your distributor is properly adjusted to the timing specifications for your engine.
While better than nothing, any specification you find is likely meant for engines in automobiles and light trucks. Likewise, the specification is a theoretical average. It takes into account various applications, driving style, fuel economy, and engine characteristics. It's likely the engine in your heavy motorhome actually needs different timing.
Here (http://tvmoparclub.com/files/tech/Mopar%20Timing.pdf) is a good article on finding out what's the best timing for your motorhome's engine. (There's at least one typo -- 'disconnect' versus 'reconnect' -- so be careful to understand what the author is trying to say.) You're probably going to need to read it multiple times to fully grasp what the author is saying.
When I put the rebuilt 351M engine in my 1980 Ford Bronco, I threw the published timing specifications in the trashcan and followed the same basic procedure as those in that article. (I also rebuilt and tuned the carburetor, resulting in an essentially new engine.) When I was done, the staff at the Ft Wainwright Auto Skills Center put the Bronco on an emissions test machine. For comparison, they also tested a new EPA-compliant automobile. The Bronco was producing 25 percent less emissions than the new car, easily exceeding EPA minimum standards.
Initially, the Bronco was getting 24 MPG highway. During subsequent tweaking, I was able to get the mileage up to 26 MPG but that setting didn't suit my occasional need to tow a relatively heavy trailer. Over several months I continued tweaking it, until I found the 'sweet spot' that suit my driving style and vehicle use. The end result was 22 MPG highway while lightly loaded and 19 MPG highway when towing a heavy trailer.
Note: I had thrown away most of the OEM smog gear, arguing significantly better fuel mileage meant less emissions. After their tests, the Auto Skills Center personnel begrudgingly agreed with me. (Being military employees, they couldn't completely abandon government regulations.)
Enough for now ... I'm taking a break before resuming my sermon.
