[freebeard]

I might have summed up better..[your doing fine].. by stating that it is almost impossible for a motorhome to equal a trailer for FE, that the purpose of a motorhome has to be that which a TV - TT [tow vehicle - travel trailer (of any description)] cannot do.

That's why I added Land Speed Record Holder to the brief. It opens up a lot of wiggle room.

So, yes, a "shop" or other where a truck chassis / drivetrain is necessary. The addition of pumps, motors and other weight-penalty additions so that several vehicles, maybe up to a dozen, can use the facilities offered by a moho.
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Screened porch / mud room: good description. Extend it to daytime animal pen (pets) and other peoples children.
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freebeard, if you state the number of nights aboard [X] for the number of people [Y] without external inputs you are most of the way there as water + fuel (propane + battery capacity) can be calculated in advance. Then a reasonable distance under best conditions for drivetrain fuel theoretical maximum. (I'd use 500-miles).

I'd be interested in a rule of thumb for fresh/gray/black water vs road gas vs home power. It would probably be of a form [x]days X [y]miles X [z]fudge factor.

Look at Class C motorhome capacities as a starter. The examples above of Rialta, Le Sharo, etc. There is a minimum which some types won't meet a realistic time (at least 3-nights, but a week would be better as the difference between two aboard and four aboard really starts to show up.) A moho for only two is getting extravagant (weight of vehicle[s] to provide shelter, water, etc.

Really, one is really no farther along than with a contractor van and portable toilet + water containers and propane exchange tanks . . in which case, why an expensive moho?
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[4] adults, [5] nights and [500] miles seems on first glance enough to design around.

As RV travel is generally in the vicinity of 300-400 miles per day (after a week or so at each camping spot) one can see how these numbers work with each other. Five nights of full independence ought to also meet a reasonable beginning for RV use in the face of disaster, etc.

Months of full independence?

In an earlier post you name-checked Bluebird. Decades ago now, in Bluegene, OR there was a school bus named Fort Home. The roof and windows had been removed from the last 6', giving it sort of a poop deck, with nice wooden door.

So how about short bus, third axle where the front bumper used to be to carry the weight of an elliptical dome—and a boattail that folds out into a screened porch. Don't get me started on the 55-gal. composting toilet trailer (the trick is you can spin the drum when it is horizontal, for mixing).