New guy here with a 120v/12v question

What year is the Starcraft?

The lights should work without a battery. However, most converters use the battery as a "buffer" for the 12 volt system. Keeps the power steady. I'm guessing something is not connected properly in the Starcraft. Curious if something was disconnected to get the lights to work? There should be fuses at the converter; one will be for most of the lights.

The problem with older converters is that they tend to overcharge a battery. Not a big deal if the trailer is not powered all the time, but the converter will cook the battery if left charging too long. I suggest a higher end replacement if you want to use the converter. A battery maintainer will work, but only when connected to a battery. (No output if it doesn't sense a battery) The maintainer will easily keep up with a few lights, but will not power a fan or anything with a motor.

Hope this helps. I'll be moving this to our 12 volt systems section.

If it's not the door switch, perhaps the 12v isn't working because the missing battery somehow breaks the circuit from the converter to the lights? Unlikely, but who knows how PO's re-wire things.

Dunno 'bout your old one, but modern converters are for people always on a hookup. They're not good battery chargers, even the "good", multi-stage ones like Progressive Dynamics. They don't put out high enough voltage (for traditional "wet" batteries, anyway), and they don't charge for long enough. Trojan and Deka both say their wet batteries need 14.8v to properly charge. Converters mostly put out about 13.6v, and they will never get you to full charge unless they're plugged in all the time because they also taper off the amps way too soon, and essentially become low-voltage trickle chargers. But since you'll always be on shore power when you're camping, battery charging isn't a big worry here because it won't be discharging while you're camping. It's more about battery lifespan in this situation.

Will the camper be plugged in with battery installed all the time? If not plugged in, and the battery is left aboard, it will probably become discharged between trips, which will really only be a problem in that it will shorten the life of the battery, especially if it is discharged and subjected to freezing weather.

If it is plugged in all year, then eventual overcharging and water loss is the thing to worry about. If your battery is a standard "wet" type, keep a jug of distilled water handy to keep it topped off. And to keep the battery healthy, buy a $5 hydrometer at NAPA, that's the only way to truly know your state of charge, and to know if you're getting a bad cell. Will you be checking on the camper/battery in between uses?

If you're using an expensive AGM or sealed battery, I'd take it home with me after every trip. You can't check or replace the water in those things, or even check the charge with a hydrometer. Leaving it in the hands of an old converter is risky and wouldn't be my choice (neither would using AGMs, but some people like them, especially battery salesmen, but that's another discussion).

If you're going to take the battery home after every trip, just keep it on a maintainer at home and don't worry about it in the camper. Ten days on a converter while you're camping won't run you out of water unless it's seriously malfunctioning and overcharging the hell out of it. Just pop the caps and look at the water after a few days and top it off if needed.

Again, since you're not depending on battery power to camp, all of this is just to keep your battery healthy. If you don't mind replacing it every year or two, it doesn't really matter. A "dead" battery that won't take a charge will probably still function as a buffer for the converter.

HandyBob's blog is the best thing I ever read about battery charging. It's loooong, but as thorough an explanation as you'll find. It's geared towards solar but includes parts about chargers and converters as well.