Is my charger dead?

Yes.
But so would 2 batteries that are dead. Or boiled dry.

If ya charger is putting out 17.5 volts.....um.....er...yep, Id say it has a problem.

I have heard about a string of incidents where these multi stage chargers have failed or got confused.

I have half a dozen battery chargers arround here..both multistage and others.....let me tell you I would not leave a battery on a " multistange smart charger" indefinitely.

I have an old industral analogue 20 amp 3 stage charger ( one of my best ebay buys ever..it is beautifull) that alows me to disable the boost charge phase.....

The fully docmented manual ( diagrames and circuit descriptions and all) warns against leaving batteries with the boost charge stage enabled unsupervised

I am in the process of building a few good old fashon transformer bassed analoge regulated chargers....because I know these I can trust.

I future you would be very wise not to trust any charger....or any battery for that matter.

There is absolutely no need to keep batteries on a charger unused for months on end.

A good healty 24 hour charge once a month should be more than sufficient.....AND.....observe that the battery and the charger are doing what you think they should....check voltage before and after.

Do this and you do two things...1. you give the battery enough charge to keep it happy and 2 you can see if the battery is holding charge long term.

Remember these batteries may be sealed and maintenance free.......but they still require supervision...if only a little.

All too often we hear of people leaving their batteries on charge continuous......then they heard off for a trip and find the battery is buggered and is not holding charge.

cheers

Are your batteries ALSO hooked up to solar????
If so check your battery voltage 3 hours AFTER the sun goes down.

If your charger is putting out 17 volts it’s had the bomb.
It’s likely that your batteries are damaged, depending on how long ago your charger failed – do a 10 to 20 hour load test on them.
Google “Capacity Testing Batteries” and you will find a procedure for deep cycle batteries.
It seems you have been unlucky – I have had 3 smart chargers float charging my batteries for many years, without missing a beat. I also have another 3 manual chargers which I use for fast charging but wouldn’t leave them connected to a battery long term.

Thanks everyone for the info and advice. Looks like the old CC is in for a hammering.

Might also be worth checking the physical shape of the batteries for any deformities? Sometimes they swell when they have been overcharged? Failing that you can remove the batteries and have them tested.