These is a checklist in the manual I posted for my 1992 here:
http://tinyurl.com/38qo4m The suggested service item intervals are on the last page (page 48).
I replaced my battery merge solenoid about 2 years ago but unless you use it extensively, I would think annual replacement is a bit of overkill. If you leave it activated continuously to charge the starting batteries, then annual replacement isn't a bad idea. I use a Trik-L-Start which automatically feeds a trickle charge (max 5 amps) from the house batteries to the starting batteries keeping the start batteries approximately .2 volts below the house batteries. There is a diode built into the unit to prevent the house batteries from discharging the start batteries if the house batteries are not being charged.
http://www.lslproducts.com/TLSPage.html.
I also use LSL's Toad-Charge to keep the battery up on my towed car even with the power usage of the Brake Buddy.