GAH Van not working on Hook up

[PRIORITY]

We have a Mercedes sprinter with GAH unit it works fine when engine running but will not fire up when on hook up.
Any ideas?

[seanf]

Do you get anything happening? controller lighting up, fans running...ect

Normally the road and standby operation both use the same electrical circuit, except on standby the mains probably goes through a transformer and a bridge rectifier. And the compressor might be powered at mains voltage.