Booster setas in nuevo

WE have just returned from a night away with the grandchildren 5 and 7. They both obviously need booster seats, we used the ones from the car which are usually fine but in the motorhome they seem to migrate. This seems to be both by the passenger seat cushion moving and the seats moving on the cushion. Has anyone else found this problem and sorted a solution?

Hi
  Just an idea,the problem may be that the seat base is not fixed and also the booster seat is not fixed,you could try fixing the seat base down with straps or a simple solution maybe to use an antislip map under the seat base and also between the seat base and the booster seat
    regards
        Alan

Thanks for the suggestion Alan.I was thinking of trying some tie down starts on the base and cushion as I think that's where the problem starts, hadn't though of trying the non -slip mats.