One way to make a detailed area specialized for tank battles, that terrain designers can paste into their overall terrain, is to make what's called square objects. These are 1 mi x 1 mi or 2 x 2 or 4 mi x 4 mi objects that get added to terrains. Examples are city objects, strat objects, towns, etc. A good example to look at is the hilly vehicle base. If you export that object to AC3D you will see that parts of it have very high resolution (like the rock face on the hill to the NW). In fact, the ground surface can have arbitrarily high resolution in a square object. In addition, you can plant as many or as few trees as you want and you can add all the water features you want and they don't have to be at zero altitude.
The down side is that it is a whopping amount of work, you have to know how to use AC3D, and as far as I can tell there is basically no documentation on square object creation from scratch whatsoever.