It's possible there will be minor issues with the seat belt pretensions connections as the double seat is not directly compatable.
The workaround is fitting emulator resistors ( available on ebay) or removing the pretensioner from the single seat, connecting, and storing out of the way under the seat.
Your single seat has a high value as they are in big demand for diy van conversions, expect around £500 on ebay. Since so many Van conversions need to dispose of the double they are available at low cost or for free.
Be aware the double seat is very uncomfortable for passengers and you will lose the ability to move to the rear from the cab.
If either of the front existing seats have swivels then you loose this facility. It wont fit the double and there is no room for the drivers to rotate. However selling on with the single seat will increase the value, perhaps to £600.
My advice is to consider alternatives before changing to the double. If you do change and have room, store the existing seat rather than selling, it will help in the future when you sell/ exchange the motorhome.
Mike