I'm no TC'er ... but it seems like for treks like that in the photos above one would want an over-kill chassis so weight was - for all practical purposes in at least dry conditions - not an issue.
Wouldn't a ton truck chassis increase water carrying capacity significantly even though camper weight didn't require a ton chassis per se ... assuming one could find low-down places to carry the water on the rig? Perhaps long, shallow-depth water tanks could be mounted up in between the frame members on either side of the driveshaft? I've read of one Class C owner who was able to increase water carrying capacity to 150 gallons with external tanks mounted up and under their small motorhome in this way. Of course in off-road conditions plastic potable water auxiliary tanks mounted up and under this way should have aluminum or steel protection plates on their bottom sides.
We have a small Class C motorhome we take boondock camping in the desert (rockhounding) that we bought new specifically because it was built on a ton-and-a-half chassis not actually required for normally expected camping weights. As a consequence we can load it up with no thought given to the added weight, so we can easily carry many additional gallons of water without even feeling it. Our current built-in water capacity is only around 45 gallons carried in a concealed tank that's mounted right down on the floor in the coach interior environment. However, I'd prefer built-in water capacity of at least double that amount, but as of yet have not added the auxiliary tank(s) for it.
Here's a link to various size RV water tanks for building in - some of the 25 gallon to 35 gallon configurations look very interesting:
http://www.plastic-mart.com/category/33/rv-water-tanks
