As long as you have #8 wire and I believe you should have #6 wire to the battery it would be fine to replace that junk circuit breaker with a proper breaker or fuse rated 60 to 75 amps.
If the breaker is in the middle of the wire just remove it. You have main fuses at the 12v panel and should have some type of fuse at the battery. This middle of the wire breaker is just trouble.
If you want a breaker I recommend a Bussman High-Amp. (manual reset) amazon has it
Or an inline fuse holder similar to
maxi-fuse-inline-fuse-holder-w-6-awg-leads-up-to-60aIn the mean time get a charger on those batteries asap.
Were you connected to utility power when all this happened? If so you could have another issue.