If you are looking at 300AMp-hr capacity you should look at a 100 amp alternator to make sure they are topped up. These batteries are expensive if your system sulfates them due to incomplete charging. Equalizing charging will not undo this kind of chronic damage. You have to count on anywhere up to 30% loss in charging system efficiency, so beware of absolute numbers.
Also get rid of the power switch on the output of the alternator and instead use an isolating diode setup. Rewire the system so that you have minimal connections in the loop: alternator, diodes, batteries, alternator.
The problem with cheaper solutions is that they are not. How much does a set of deep-cycle batteries cost? If you don't build in adequate charging capacity, if you don't use a proper three-stage regulator, if you don't have adequate battery capacity to keep above the 50% discharge level, you will be replacing batteries more often than you need to be.