We were also there in July / August and since it’s a “dry cold” (like Eric Veenstra noted) the freezing temperatures aren’t a big problem as long as you have a good sleeping bag. The same holds true for the wind.
For us the big problem was SAND! It’s the stuff of nightmares!!!
While researching your route you probably came across Harriet and Neil Pike’s website
http://andesbybike.com. In the Bolivia section they write:
Road Conditions
There isn’t much tarmac in Bolivia, so if you decide to keep to any paved roads you find, expect to be on a major route and sharing it with plenty of traffic.
Unpaved routes in Bolivia have a reputation for being tough on cyclists with many corrugated and sandy kilometres. While there are certainly some poor surfaces on the Bolivian altiplano, there are also plenty of quiet unpaved roads with surfaces which are fine to cycle about on.
Sure the unpaved roads are “
fine to cycle about on” if you are as thin as Harriet or Neil Pike - see photo
https://www.flickr.com/photos/pikesonbi ... ateposted/ and carry an absolute minimum of gear. If you weigh more, than be prepared to spend hours / days pushing your bike and staring at the ground – trying to find the ‘best’ sand track to push your bike through.
For instance on one of their shorter routes they wrote: “
How much we had to push on this route?” “Not at all.”
For us (and another Dutch couple we met) we spent more than half of the time - on the same route - pushing them through the sand.
Afterwards I contacted the Pikes and asked them about their experiences with dry (non-compacted) sand. They replied that sometimes they attach their mountain climbing harnesses to their bikes to aid in pushing them.
https://www.flickr.com/photos/pikesonbi ... ateposted/
Unfortunately it’s a bit difficult to find out which non-paved roads are good to cycle and which ones are pure “sand traps” since a lot depends upon how recently the road has been graded.
If you want to experience what it’s like to cycle Bolivian backroads beforehand - then take your loaded bike to Zandvoort and on a dry section of beach next to the dunes - start cycling.
The Salar de Uyuni is fantastic as is the Salar de Coipasa and Sajama National Park.
Cycling and camping on the Salar de Uyuni
Sajama National Park
As for Potosi – I can’t say we were impressed although we didn’t take the mine tour.
) 
