I have a shumacher ship to shore charger. Its like a $50-60 charger and was wondering should I be leaving this thing on all the time? Its what I have been doing but this unit never shuts down and the fan is going constantly and I am worried the charger is gonna burn out prematurely.

I have one of them that I used to use.  It too seemed like it never shut off.  I was concerned that it would boil my batteries dry so I got a hydrometer and checked the cells.  When they showed full charge I would unplug the charger.

I now have an on board 3 bank and a mfg rep told me that it shouldn't be left on for long periods of time.

If I had a rep tell me I shouldn't leave his brand onboard charger pluged in for long periods, I would have walked away leaving him holding it before he go the last words out of his mouth.   Any good onboard should be plugged in anytime the boat is not being used.  If the boat only get used once a year, the charger only gets unplugged once a year.

I have three installations using that Schumacher charger that have been on constantly now for many years maintaining deep cycle UPS backup batteries.  The batteries have required no maintenance at all.