Hi Have to agree the best surface to run on is soil , nice and muddy if it rains(weathers the tanks) , But if you want a all weather surface that dosnt change , if you can get hold of crushed sandstone (I suppose it depends on where you live) here where I live its cheap its used a lot for drive foundation's .You lay it around 2-3 inches deep and compact it sets when it rains into a hard wearing surface(put plastic down to stop weeds coming up you can also water with weed killer) , you can run tanks on it and no problem with it jamming the tracks and no muddy foot prints when you go in the house.
regards pete