It's hard to say from here. Yes, it probably will depend on how long the run is.
Did you run it when you bought
it? If it ran ok then it's likely that the pump is good.
These pumps don't create much suction or pressure even when new.
I know people do run them from main tanks but I'm not sure about the best way to tie in because I've never tried doing that.
I think "T-ing" into the line that feeds the main outboard and using a separate, dedicated primer bulb might be what I would try. Just a guess though.
Before making changes, be sure that the hose and fittings are tight and in good condition so it's not sucking air.
The primer bulb check valve works best when the flow arrow is pointing somewhat upwards towards the outboard.
Check that the quick disconnect is solidily on and that the latch is properly locked.
Grab that remote tank if you still can.
Good luck.