Sure you can have it off center, but I would try to keep it as paralelle as possible with the side of the RV. Having it sideways to the wind can cause a side pressure, so a inch or two that it is sideways will not matter, but to strap it down so that it is one foot closer to the drivers side on one end or the other would cause the side that is leading into the wind to have a lot more air pressure on it.
You are talking about 30" hang over on each end. That seems like a good idea. If you have more than 40" hanging forward, I would guess it will be getting close to the tow vehicle on a steep gas station driveway? Probably better to have it equal. You will need a red flag on the back of any load that is hanging more than 12" behind the bumper, so flag it anyway.
Fred.
