Without knowing what your bus weighs, comparisons are a bit meaningless..
Well yes true, it does have an affect, but for long distance crusing, provided the tyres are up to pressure and not knobblies, the type of body, wing mirrors and appendages will dominate fuel consumtion above 45 or so mph... we all tend to run fairly heavy when touring abroad, which is what most of these posts are referring too, certainly we're all 2t or over. Also it wasn't a particularly hilly drive, although the last section to Mammut were pleasant, but quite slow A type roads with lots of villages (slow, slow, quick, quick, slow) where weight would predominate.
As for your Silverbullet at 3600 rpm, here some figures I've extracted from a set of SFC curves I derived for the subaruvanagon forum a good few years ago, for the EJ22, using an SFC polar published in an SAE paper. We can maybe make a few adjustments based on improvements to the engine in later (Post 1990) models and to transmission and running-gear inefficiences (or simply assume they cancel out) and also speculate on speedo/tyres etc later too... but for now...
The bottom line is:
BHP(flywheel); RPM; MPH; MPG
30 3600 80 38.517
40 3600 80 31.028
45 3600 80 28.504
50 3600 80 26.347
60 3600 80 22.680
70 3600 80 19.895
80 3600 80 17.777
90 4000 80 15.760
So it looks like at 3600 rpm, only less than 45 BHP can get anywhere near 30 mpg at 80mph for this engine...
The question then is, can a T25 (however sleek) attain 80 mph on 42 BHP (at the flywheel), that is, maybe from somewhat less than 40 HP at the wheels?
If we consider the power output of various new models as road tested or specced by VW, and their tested/quoted top-speeds, model for model, we should then have a good plot of required flywheel power against speed and maybe can then draw some conclusions.
Personally though, I'd say 80 mph from 40 BHP or less is not possible, more like 70 would be my instinct, but reserve that big differences would apply to body style, tyre tread, Syncro or not, as well as weight.
As a guide, the aerodynamic drag alone approx. doubles from 56 mph to 80 mph, which might be half or more of the total at 56 mph, thus perhaps being three-quarters at 80 mph