I had something similar on a 2000 Pace Arrow but mine said Pump On and Pump Off. It was for the demand pump. I don't know if that is the case here but worth checking. If you were not running water when you flip the switch or were hooked up to city water you might not notice.
I'll take your word for it, but it makes no sense to me why the fresh water pump would be labeled "Dump", and not "Pump".
I was just guessing, but I would have thought it might be something like a macerator sewage dump pump
or perhaps one of those two tank set-ups where one tank has a macerator pump that transfers to the other tank for gravity dumping.
Why not flip the switch and see what happens?