find out from the manufacture what the 20 hour rating is, and apply that amount of load, if the battery is good and fully charged it should carry this amount of load for most of the 20 hours.
true deep cycle batteries should not be tested with a load tester made for starting or marine deep cycle batteries (which are more of a compromise between a starting battery and a true deep cycle).
the thick heavy plates of a true deep cycle are not designed to deliver a large amount of amperage for a short test like that which a automotive load tester applies.
get them cleaned up, and charge them until they max out the specific gravity (no further rise after about 3 hours) and then let them rest for 24 hours.
recheck the specific gravity and see if all the cells match up pretty close, if not you may have to do an equalization charge and then rest them, and recheck the specific gravity. if after all this the specific gravity is very close cell to cell
you can then do a 20 hour test to see what you have.
you will want to be armed with all the info you can get from the manufacture, what is the specific gravity of a fully charged battery? what is the 20 hour amp rating?
what is the end voltage spec supposed to be after a 20 hour test?
an automotive tester can be used to cull out the worst of the bunch after a good clean up, fill up and charge,, if the battery falls on its ass with a hundred amps applied, it might not be worth the effort to continue with.
or if you have the time and the inclination to do so, you can play around with whatever alternate methods you like to try and recover the poorly performing ones.
typically though you will find a few weak links that will only drag down the good ones in a string so it might not be worth the effort.
bob g