Re: Electrical questions
The voltage rating on switches is based on the insulation - higher voltages (electrical pressures) are more likely to arc over (rupture the 'container') than lower voltages. The current ratings are based on ability to avoid heating - even fractions of an ohm will cause significant heat at high current flows.
DC is tougher on switches than AC. In AC, since the current is changing direction 60 x 2 times per second, the electrons all don't actually have to cross the contacts, as long as they can magnetically influence the electrons on the other side to keep changing direction in sync with them. This is how capacitors work. In DC, the current flow is one-directional, and all electrons must actually cross from one contact to the other. Any poor connection will cause heat with high current, heat causes failures and/or fires.
Up to about 3 amps, I wouldn't worry too much. One switch for one light should be fine. You can even use failure-prone Radio Shack switches. Start running strings of marker light bulbs for illumination at 2-6 amps each, and we are talking currents enough to stress weak connections and components. That's why you usually have a solenoid in the bus to handle the accessory current, instead of running all the accessory power through the key switch. Check for DC ratings for switches used for higher loads, and if possible try to leave a 50% safety margin (i.e. use a 30-amp switch for a 15 amp load).
Yes, you will have to have a means of switching from shoreline to generator and back, whether you have an inverter or not. This could be an automatic transfer switch, a manual transfer switch, or a socket to plug the shoreline into the generator. If the generator is permanent, you could have a cabinet with two outlets instead of a switch, and move a plug from an outlet off of the shoreline to an outlet off of the generator feed.
Reminder: Do not tie the neutral and ground together inside the bus when running on the shoreline. The link is provided by the campground/house providing the power. The difference between RV inverters and residential ones is an extra contact to break the neutral to ground link when external AC is available, and connect them when AC power is coming from the battery. When on generator, the generator wiring should provide the neutral to ground link.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.