it's probably a solenoid for the connection between the Chassis battery(s) and the House battery(s) for the 'charging' flow, or the use of the 'AUX' or emergency start switch, which ties the two battery bank together also, momentarily.
The solenoid is controlled by another relay that senses when the Chassis batteries have reached a certain charge, by the Alternator, and switches to connect the two battery banks, sending the 'extra' power to the house batteries, and vice versa when you are plugged into 120v shore power, or the generator.
I have both a Trombetta, and a White Rogers.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada.... formerly Georgia, then NorthCarolina, now Alabama : )