Yes, you can drive a motorhome with a full water tank. However, it's important to remember that the extra weight can affect the vehicle's handling and fuel efficiency. The water in your motorhome water tank can add hundreds of pounds to your vehicle's weight. This can make your motorhome feel sluggish and harder to steer, especially on hilly terrains. It can also increase your fuel consumption. Therefore, while it's completely safe and possible to drive with a full tank, you might want to consider only filling it up when you're close to your campground or destination.