When you put the new battery in place hook a cheap digital ammeter (wal mart, harbor freight, sears) in series with the + line and measure to see if there is any current flowing. Use the 10 amp DC scale on amps.
Yes, the "charger" under the bed might be causing the issue but so could a lot of things. There were no parasitic draws on the old trailers but some of that vintage had a built in circular fluorescent light with a kind of a crude 12 volt
ballast converter. If it was switched on, but didn't light the light, it could still draw power. I have had a couple of dead 12 volt
fluorescent lights which did that. Defective and did not light, but if the switch were left on, they still took power.
The battery you got (long and flat) will most likely not be a deep cycle one. You are lucky to find even a starting battery in that shape. Consider looking for another location for the battery so you can put a deep cycle type in the next time you replace it. You also should consider a modern 3 stage converter/charger such as a Progressive Dynamics PD9245 to keep the battery properly charged.
And certainly someone in the shop could have turned on some item and never turned it off, killing the battery.
A fully charged battery, in Oklahoma winter months will not self discharge to dead over a 6 month period with no load on it. I leave mine, totally disconnected, for 5 winter months with no recharge, in place, in both of my trailers with absolutely no issues.