congrats for your choice of Mongolia, you'll love it.
Camping in Russia: I didn't see one campground. The "tent" signs actually direct to car parks where truckers sleep in their cabins. You can camp in the wild but beware of the dreadful mozzies in Siberia! The bikers community is very friendly though, they would sure help you in this department.
Mongolia weather. In summer you can go from 45 degrees to 0 degrees in a couple days so plan for everything.
To see: not much in particular, but I remember Amarbayasgalant monastery, the place is beautiful, and the road is not bad with just a couple river crossings at the end. If you prefer the desert, the area around Yolin Am is great to ride, but rougher as well. Basically, the north is hills, grass and trees, the south desert, rocks and sand, but everywhere are nice people and fun roads, you can't go wrong.
Maps: that's right, there is a nice road atlas of Mongolia, you can find it everywhere in Ulaanbataar (there's and English version although the Cyrillic version is easier to use). It is pretty detailed, but that's useless as most information is wrong or outdated. The thing is, besides the 2 sealed roads, most other "roads" move from one year to another as the original one deteriorates and the 4x4 drivers "create" new ones. So use your intuition.. that's part of the fun of going to Mongolia!
A GPS is not that useful for the same reasons: if you have punched in the GPS coordinates of your destination (from the LP or posted by others here) it'll prevent you from getting even more lost, but it won't lead you back to the road. When I was there I found that the little roads that are on the Garmin world map gives you a good general idea, don't expect more than that.
Oh and did I mention that the locals are useless? they will point you the geographic direction (which you have on your GPS), good for their horses, not great for our bikes.
Range: 25l should be OK, although top it up everywhere you can. There are surprisingly many jeeps and motorbikes in Mongolia so if you ask around ("benzin") you will find something (they ma ask you the same and I did occasionally help some locals with some of my 28l tank).
At the end of the day I found it much easier that I first thought, and a lot of fun. Enjoy!