Something to think about on test drives is the weight of the unit. When I bought my DP I found out that all the tanks were full of clear water. The filled both holding tanks to capacity as well as the fresh water tank. (The only thing they didn't fill was the diesel tank) The reason they did this is because if you drive a DP completely empty of your gear and with all the tanks empty, you can get a real rough ride. You'll feel every bump. A DP rides smoother with the weight in it. I would think the same would apply to a gas unit. So if the ride feels bumpier than you would expect, it may be because it's completely empty.