Its all about the current. The system is fused according to the current load of the size wires in the fuse panel so if you have too many goodies on the power bus you'll know when you burn the fusible link. Of course, you can add your own power panel tied directly to the battery but if you add any wires to the bus you must fuse accordingly or you'll end up with melting wires while driving.
There are two batteries in my Chevy 2500 with a 130+ amp alternator which can charge my Montana's batteries as well as power all the lights, blowers, radios, etc. inside the cab. Most modern components like a GPS or portable DVD player have a relatively small current draw so this is not a problem.
If I were really concerned about the amps I'd order the new Ford Super Duty with dual alternators. You'd be able to power all those gawdy fog lights, charge a 6-cell golf cart battery pack in your Montana while running a microwave in the back seat!!!
Remember this, if you blow a fuse do not replace it with a larger one