When I bought the trailer it was plugged into 110, when I was looking everything over I pressed the button on the deal (don't know what it's called) that shows fresh water, black, gray tanks and battery levels. All the tanks showed one light, and the battery level showed all four (I think there's four) lights, I'm assuming with all four light on the battery would be charged? When we unplugged the 110 I tried to operate the slide.....nothing. I pressed the button "deallyl whopper" again. Now with the trailer not plugged in, the battery indicator only had one light versus the 4 before. This along with the slide not running lead me to believe the battery was never fully charged as it showed when plugged into 110. We plug the 110 back in to retract the slide.
Thanks - That helps quite a bit. The deally whopper is usually called a monitor panel or monitor gauge, since it is used to monitor the tanks and battery state.
When plugged into 110 power, the battery monitor gauge is showing the
combined status of the converter/charger and the battery. The charger is pumping power into the 12V system and attempting to charge the batteries and the gauge (which is merely reading system voltage) shows that. Full voltage in the system does not necessarily mean the battery is fully charged, but low voltage definitely means that it is not. When the converter/charger has 110 power, it effectively supercedes the battery and supplies 12VDC power for all needs and the gauge will normally show full battery power available. Thus your slide operated on converter output, not battery power.
Your truck's alternator apparently has a charging line back to the trailer (it's optional - not all do) and it appears to have charged the batteries somewhat. However, there is a common phenomena called a "surface charge", in which the battery gets enough juice to show some reasonable voltage but cannot supply power at that voltage for more than a few seconds. Any large load (e.g. the slide motor) depletes it almost instantly. The fact that two hours of driving resulted in only 3 of 4 lights makes me suspect that the battery wasn't accepting much charge from the alternator either. Two hours of driving/charging won't provide a full charge, but it should show full system voltage and thus 4 lights.
Your 94 Wilderness undoubtedly has one of the Magnetek Model 6330 30A converter/chargers that most trailer manufacturers used in that era. They are mediocre chargers and probably put only about 5-6 amps/hour into the battery. That's not much. They are typically integrated with the 1110V load center (breaker panel). You can't tell a whole lot by testing the converter/charger output voltage - it merely tells you that it is turned on and at least basically functonal. Simple voltage checks won't tell you whether it is doing a good job of charging or not, but at least you know it isn't totally dead. The fact that it is powering the slide is also positive - it is probably producing its full output (such as it is in that model).
Your trailer shows all the classic signs of a bad battery(s). It won't take a charge and won't power any substantial load by itself. I would pull the battery(s) and have it tested under load. If you don't know anybody with a load tester (about a $40 device), most auto parts stores will do it for free. Unless the battery comes thru the load test with flying colors, replace it with a good deep cycle type (not an automtive starting batery).
I would not buy a new converter/charger, at least not yet. Assuming you do have the integrated Magnetek converter/charger, replacing it with a standalone converter such as the Progressive Dynamics is more than a simple remove & replace operation. However, Manetek (now called Parallax Power) makes an "upgrade replacement" unit called the 7300 which should slip tight in and provide a sustantial improvement in both quality and output. I think it may still be available. Here's one possible source:
Eco-Vantage Catalog.
The Magetek/Parallax Power replacement is no where near as good as a PDI or Iota converter, but will be a whole lot simpler to install if needed. However, I'm betting the seller lied or was simply wrong when he told you the battery was "good".