Electrical involving auxillary battery replacement

That battery should only be in use during heavy loading on the primary battery ... Does your owners manual say that both batteries should be the same rating? If the deep cycle voltage is just a fraction higher (ie; primary is 13.3V // deep cycle is 13.8V) then the deep cycle will be drawing more load than the primary (unless there paralleled) I suspect a regulator issue

Thanks. Nil in the manual about this but I will look into the voltage differences

A bigger battery is usually better unless you continually run it down while parked ,,, because alternators have a maximum output that could be exceeded if the larger battery draws a lot more current while re-charging