You said "the draw is obviously weak, " which indicates that the problem is in the wiring
or connections, not in the toilet. The most common reason for "sluggish discharge" is low voltage to the toilet. That can be due to a weak battery
, corroded connections, loose wires. Start at one end of the system and work your way to the other end, checking 'em all.
You have another toilet that's working fine...are they both on the same circuit? What else is on the same circuit as the troublesome toilet? (If your answer is anything BUT "nothing," whoever installed your toilets didn't know what they were doing, 'cuz electric toilets, treatment devices etc should always be on their own dedicated circuit, shared by nothing else that can pull power away from it if on at the same time)..'cuz operating at low voltage can not only cause sluggish discharge, it'll destroy the toilet motor
If it's not low voltage, it may be a head discharge line that has enough of a buildup of sea water minerals to all but block it.