How do I drain my deep cycle batteries?

No you don't need to drain them. True, the point about deep cycle batteries is that you are much less likely to do them any harm if you do allow them to fall very low on charge than you are to regular (starting) batteries. (The plates are much heavier and somewhat further apart). But it is still best to avoid if possible: It still doesn't do them any good. Periodic full discharging of batteries - called 'calibrating' - is more usually reserved for the realms of nickle-cadmium and lithium-ion etc: The type that power mobile telephones and laptop computers and other such devices. And it's of much lower importance on these even, if they are always kept fully charged. Just keep them regularly topped up as you would any other. Whether in regular use or not. Batteries that are kept fully charged have much longer lives than ones that are left fully or partially discharged for long periods. -|--)

there's a lot of myths out there about deep cycle batteries. no need to drain them...in fact, like all lead acid batteries the shallower the average depth of discharge, the longer they last. Also like all lead acid batteries, trickle charging is not best at all. Use the highest setting on your charger, not exceeding 25% of the AH rating of the battery. 10 or 12 amps is a good all around choice for any battery. Any time spent at less than 100% state of charge will shorten your batteries' life, so recharge 'em soon as possible after each use.