Thetford N80 3 way fridge?

We have this damn Thetford N80 3way fridge that recently after using on electric hook-up now does not want to stay alight on gas. Have cleaned out everthing we can think of. Lights first time every time and on very rare occassions will stay alight, but most of the time as soon as you release switch the gas goes off, very frustrating as going away soon for 2wk holiday in a field so gas needed. can anyone help? Tried some of earlier suggestions that have been posted but not helped........does this mean a new fridge.........!!!