Almost every glider has just one wheel close to the centre of mass. Some have a small tail wheel, school gliders may have a front wheel that avoids to scratch the nose when you are learning. Whatever the number of wheels is, you spend most if not all the time balancing on the main one.
Every skilled glider pilot is against anything that could create even the minimum amount of drag and is not strictly necessary!
The key point here is the amount of fuel inside the wings, which is really tiny (a small motorbike tank per wing) and allows for safe landings. If it was more it could create problems mostly because of its weight and the related stress on the wings when touching down. Indeed most of the competition gliders allow to fill their wings with water in order to increase the performances at high speeds, however it is mandatory to evacuate it before touching down, and of course you wouldn't want to do the same with unspent fuel.