Have the same compressor mounted in the bed and have had issue with the relays rusting out. Water in the air compressor. Don't recommend mounting under the truck
I assuming you are talking about the ARB twin? If so I am considering remote mounting the two air intakes up under the bed rail bed hind the builtright bed racks to help with keeping them away from the elements a bit more.
On the relays did you replace them with a more weather resistant relay or factory?
I am likely going to switch to a vertical tire carrier/bypass rack which then I’ll need to permanently remove my bed cover.