Most dual battery setups are connected in parallel,or together.So the are one in the same.They charge together,and discharge together.It really doesn't matter where you make your connections at all.It may be easier on the second battery,as they only have one battery cable on each post,unlike two on the main battery,where the battery cables are joined.You can get battery bolt extensions for adding additional items.If your accessories run the batteries dead,then they will both be dead,and the vehicle will not start.Because the batteries are conected together,the voltage regulator just see them as one big battery,and they are charged together at the same rate.
If you have an isolated system,then the two batteries are isolated,and are connected separatly to different devices.Usually one for starting,and one for accessories.If the accessories draw the one battery dead,then the starting battery will not discharge,and can be used to start the vehicle.A battery isolator is used to prevent one battery from discharging the other,and to determine the different charging rates for each battery.It will usually charge the main,or starting battery first,then the aux battery.
The parallel setup is best for a plow truck,as you need the extra reserve capacity that two batteries provide.An isolated setup would not function well for this application.Your GMC should be a parallel setup,so your fine.