A modified tank is frustrating to run through grass. They run through it because of the added weight, not over it like a lightweight stocker.

The Pershing and Walker because of their rear sprocket act like lawnmowers, sprockets quickly loaded with blades and roots. This can be alleviated by adding weight to the front increasing traction across the treads. Still it's better to run in dirt of some kind, more realistic and fun too.

If you have enough space, mark off an area that won't get you divorced and lay plastic down. Then go to a nursery or soil company and check out what kind of dirt they're selling. A couple truck loads of whatever you pick should do the trick, don't be afraid to haggle on the price.