Dual Battery system problem

We have a dual battery system consisting of a 150Ahr deep cycle battery in the
back of the car, and a 80 AHr cranking battery under the bonnett and a 100Ahr
deep cycle battery in the camper with a 140Amp isolator under the bonnett and a
100 Amp circuit breaker between the isolator and the batter in the back of the
car.

On a recent trip everything was working fine until day 6, and after a day of sand driving, when the fridge in the
back of the car showed the temp rising. The problem seemed to be with the
battery in the back of the car. Took it for a drive and it was ok, but after a
few days, charge didn't hold for long.

After we got home further investigation showed that the isolator under the
bonnett was switching off and on rapidly, so we disconnected the earth and
reconnnected it, and now it doesn't go off so the two batteries are connected
constantly.

Any advice as to where to go now? Does it sound like we need a new isolator?

Please be patient - I'm only the computer operator translating for hubby. Thanks in advance.

Jeannie and Doug