How I did it: In my case the fuse panel was well located so I put in a 20 amp fuse (Last step) in a blank slot and hooked up some 10GA wire to a pair of Anderson power Poles.. I use a lot of those.. The standard 2-pole automotive connector is a good alternate choice. You put the NEGATIVE lead (motor home end( on the exposed pin.. that way if it shorts to ground, well. its already there.
(This is not an issue with power poles).
In your case. since you need the pump outside, I would either enter the RV via the electrical bay or simply put a short, FUSED lead with either power poles or flat-2 Automotove connector pictured by a prior response direct to the batteries (unless the fuses are closer)
alternate route.. You can follow the water lines up into the interior, and likely follow them till you can branch away to the fuse panel. (How I do it). if you wish as well.
Whatever you do. FUSE IT.