I replaced my submirsable pump with one outside the tank (sounds like what
you have already)  I found a repalcement at an RV place for $35 or so.
 Unfortunately, I have never gotten the hoses to reseal totally.   Have to
suck/siphon the sink when I want to use it, as there must be no air in the
system for it to work.

Also, check the wires with a voltmeter to see if current is there, if the
replacement doesn't work.

Peter