I've read an account of this being done in a hoop house.
They set up the woodstove ,couch, etc.
Summer time, I think they covered it up with shade cloth.
I think they even kept a few ducks in one corner over the winter.
With a woodstove, it
should be dry during the winter, and if you open the sides in the summer, no humidity will build up.
Are you going to isolate your slab from the ground with insulation?
If so,consider using your slab or your back wall as a mass for a rocket,or even just blow solar/woodstove air them.
You have well
water, so an evaporative cooling mister might be an option, or an air to water heat exchanger (heater coil in front of a fan) or first one, then the other, then the runoff irrigates the dedicious shade plants...
If you ever decide you want a more solid roof, the existing
greenhouse structure could be a good framework for ferrocement walls/roof.
If you have a reliable roof above the RV roofs, you could use those roofs to dry food or
firewood or laundry,or grow food up there.
A
trellis for vines could be good year round,a living
solar system.
Maybe it's to much to wish for, but a slab with a functional greenhouse/shadehouse onto makes me want to add a rootcellar underneath.
Maybe
berm the back, one side and a few feet up the front,leaving one side for adding and removing vehicals.
Coukd you use vapor permeable housewrap to isolate the soil in those berms?
By driving the water vapor out during the heating seasons, the soil would become more and more insulative .
A 3-6" layer of topsoil and ground cover could protect the vapor barrier.
A second impermeable barrier would start level with the bottom of the foundation and slope slightly down and away.
In between ,a sandwich filling of slowly drying earth.
OK, so
enough with the crazy ideas,but I think your base idea is sound as can be.