Yep, if you do their resistance test, it can be relatively painless to discover what the problem is. Although in our case (sister brand to your Mainship), there's also an in-line fuse on the senders. 1 amp, I think... or maybe 3.
OTOH, if it's the sensor... WEMA used to use plastic sensors for freshwater and holding tanks. They apparently crapped out
fairly regularly. For example, ours were both non-functioning when we got the boat, only after 3 years in service.
The new sensors are stainless, and assuming you have access, the swap is relatively easy. Even for the holding tank, in our case, although some serious flush/rinse/pump/repeat action is recommended first.
BTW, if it turns out to be the gauge itself... although that might be unlikely... I'd probably recommend to replace the display with a CruzPro digital tank gauge... which should work with the WEMA sensor.