Depending on the age of the tank, it may hold up with some plywood, but I agree with everyone else, why chance it? I have come to the point where laying plywood to get the truck to the tree is not worth it, unless it is not safe to climb. Not only from the time is money standpoint, but about a 6 weeks ago we had to pull the truck off the driveway into a back yard only in a residential neighborhood. Only had to go back about 30 feet to get to a medium sized ash tree over a garage that showed some nasty rot at the bottom. No adjacent trees tall enough to tie into either. We pulled the truck in right where we wanted it and skipped the plywood because the homeowner is on city sewer and we had about 2 months without a lick of rain. Got the tree down without issue. I jumped in the truck and backed over the same spot we pulled into. I only moved about 3 feet and the entire front left side of the truck dropped to the axle. Long and short of it is that we got the truck out of what was probably about a 100 year old septic tank that no one knew was there. Why did this change my mind so much? Because you just never know what the heck is buried out there. If we had setup on the tank not knowing it was there imagine if it gaveway while someone was in the bucket!!!!! Obviously it held on the way in, but not on the way out. Even if we had laid plywood, I don't think it would have helped because most septic tanks are bigger than 4'x8' and the weight just will not dispurse. Just becareful out there!
Luke