I've recently installed a 12V 100 AH AGM deep cycle battery in one of the storage lockers of my 2001 F-550 7.3L PSD truck. The purpose of the battery is to power small tools and appliances (read coolers and boom boxes), and recharge tool batteries when we're in out-of-the-way places where there is no "shore power" available.
Right now I'm charging this battery from house power overnight. It's desirable that I be able to recharge directly from the (original equipment) truck alternator when we'retraveling.
A friend of mine (ex-Army) who is knowledgeable about medium duty trucks has looked at the alternator and advised that I should be able to safely connect the deep cycle battery to the truck starting batteries in parallel, with a "T-handle" disconnect switch in the circuit to separate the deep cycle battery from the truck batteries when we're using it.
I'm still concerned about possibly overloading the alternator and possible complications with direct-connecting a deep-cycle battery in parallel with dissimilar 12V starting batteries.
Any suggestions, comments or cautions would be appreciated.
Regards, John Kinney