Securing our Dogs

We will be travelling to Spain in our new motorhome soon. We only have seatbelts for driver and passenger, so can't secure them via these. The layout is bench sofa seats behind the cab, and kitchen at the back. The floor is not wide enough for a crate! Also, our eldest travels far better when he can see out of the window, so has sat on the sofa seat so far. An option I thought of is to have the main connector for a seatbelt fitted to each seat somehow.  Has anyone done this, is it possible?  Or can anyone suggest any other options?  The seats convert into the bed, so a crate could be put on one of the seats pulled out, but I still can't secure it. Any help / suggestions would be lovely!! Thank you