Lazy Daze Owners' Group

Our 1983 LD got about the same MPG but at a 5-10 MPH higher cruise

I would check all the previous suggestions and add this one.

The G-30's 350 V-8 suffered from distributor problems. Poor lubrication will cause the centrifical-advance shaft to seize.
This limits or eliminates timing advance as RPMs rise.
As the vacuum advance canisters age. they develop leaks and do not advance correctly during cruise conditions.
Either of the above conditions will severely impact economy.

Changing to dual exhaust, with a crossover pipe, will add power and economy. Many older LDs had this exhaust system installed immediately after purchase at a shop near the Mothership.
Dyno-tuning was also a popular post-purchase enhancement. It mostly involved advancing the initial timing and changing the centrifugal advance springs, so the timing advances quicker.
You can do the same with a $10 assortment of springs, a timing

a lot of patience and dedication to the scientific method.
Engine tuning is becoming an lost-art due to the lack of easy adjustability in today's modern computer controlled engines.

Larry