This Could be an ESV (Electronic Spares Vehicle) the generator is used to power this and the ERV (Electronic Repair Vehicle) which was in a container that ran the full length of the flat bed.
It could also be a command vehicle (can't remember the TLA, there must have been one) The generator would then be used to power the switch which was the centre of the trunk node as previously mentioned. Think of the switch as a telephone exchange.
Don't ask me too much about FV439's I was at the back, sinking in Belgian mud!