Is this the relay? If so you can get it on Amazon or any number of RV suppliers including Suburban I believe.
https://suburbanrvparts.com/suburban...2-16-d-de-del/
Why it's there I have no idea as it just complicates the operation of the water heater IMO. I suppose it's just to make sure that you are "really" sure you want to turn on the water heater. The switch inside the trailer activates the relay (12vdc) which in turn sends 120vac to the switch outside on your water heater. When you turn that switch on it then sends 120vac to your thermostats and heating element. They burn up occasionally from what I've read and I carry a spare in my spare box.