You need to find out what these isolators are supposed to do.
It sounds like the battery isn't being charged. An alternator problem possibly or something to do with these isolators.
You need to work out what they are supposed to do.
Are they OEM things? Have that been added as part of the truck being made a tanker? Ar they just something that's been added for some reason?
There are specialist forums for DAF trucks. You'd be very well advised to seek some out and post there.