The semi truck units I'm familiar with were all lubed by the engines oil. Nothing in the world says that it has to be that way though. If it has its own lube system/sump, than change it regular. I expect than even a new unit of this type will get a small amount of diesel in it. I'd watch that real close. Since the oil you references had been there forever... the fact that it did have some diesel in it may not really indicate issues. As the man said though, change it out, watch it like a hawk. If after a few hours runtime your still getting significant diesel in it... you may need to service it. If its only a tiny trace amount... I'd probably just keep using it.
The amount of wear may or may not be as big as you think. If its a slow running engine or if routine operations were slow ops, then there may not be as much wear as the above gentleman suggests... fuel oil in itself in a fully immersed operating part at slow speeds may not have worn it all that bad. Hard to say... just how black or nasty was the oil /fuel mixture you drained? If its isolated from the main crankcase... it should have remained fairly clean. Was there any metallic filings/shavings present (trace amount would be normal). Was the oil dark with non-oil contaminants (metal or otherwise)? If you showed reasonable sign of foreign materials at all, and it truly is isolated from the crankcase oil system.... then I say it will have internal issues. If the oil was clean and only had fuel oil present... I like run it a bit and see.