Which battery isolator relay

Hi all,

I just bought a 1992 Jamboree on a Ford E350 chassis. I noticed that the chassis alternator will not charge the aux battery.

my understanding is that the left switch in the picture is:


down - aux is disconnected from chassis
middle - aux connected to chassis when ignition switch is on
up - aux connected to main for emergency start.

Is that so?

If so - sometime it works and sometimes not... I am almost sure that the battery isolator relay is defective, and it is not the original.
How many amp should the replacement relay be? 80Amps for $20? 200Amps for $50? more?

Help is appreciated,
-Guy