power not "on" when using battery

Letting the battery voltage drop below 12.00Vdc(50% charge state) will shorten the life of the battery.
When connected to shore power, the voltage at the battery will be 13.2 to 14vdc being charged from your converter/charger. After a healthy battery is fully charged, at rest it will read 12.6+ vdc.
Here is a 12v battery voltage State of Charge chart:
12.6+ = 100%
12.50 = 90%
12.42 = 80%
12.32 = 70%
12.20 = 60%
12.06 = 50%
11.90 = 40%
11.75 = 30%
11.58 = 20%
11.31 = 10%
10.5 = 0%

Chad/Jen wrote:
The posts on the battery read 11.8 with the voltmeter. I did notice that the negative washer and nut is rusty, but I didn't think that's cause an issue.

That is a nearly fully discharged battery. Clean your connections. Check for voltage on both sides of positive cable circuit breaker (close to battery).

Duplicate

Chad/Jen wrote:
The posts on the battery read 11.8 with the voltmeter. I did notice that the negative washer and nut is rusty, but I didn't think that's cause an issue.

That's pretty low....believe it or not.

What happens when you plug in your trailer....does that reading change? And after 12 hrs plugged in?

The posts on the battery read 11.8 with the voltmeter. I did notice that the negative washer and nut is rusty, but I didn't think that's cause an issue.

A reading with a voltage meter is really needed to pinpoint the problem if it is not a dead battery or blown fuse. Places to check voltage are: battery, auto-resetting circuit breaker near battery, main 12vdc fuse in panel.

Check your battery, or have it checked to make sure it is charged.
Are the connections clean and tight?

Sounds like the converter is working if you get 12V when plugged in...
Since you have no 12V when unplugged: Is the battery dead (use a voltmeter with the TT not plugged in). Is there a battery disconnect switch?

sounds like a dead battery and/or non functioning charger
check the breaker for the charger first and battery voltage