Something is turned on and/or your battery isn't fully charged.
The "standard 80 amp hour," 140 minute reserve capacity deep cycle battery that is probably on your trailer tongue really only has about 58 (not 80) useful amp hours at the discharge rates in campers and boats and the remaining are when the voltage is too low to run anything. One of these batteries is lucky to last the night, especially when indifferently charged. A 10 amp battery charger can't possibly fully charge an 80 amp hour battery in 4 hours. At best you can put back 10 x 4 = 40 amp hours and only if the current flow stays at 10 amps. Since the current flow reduces as the battery voltage increases it's best to plan on 24 hours to charge this battery.
The best your tow vehicle do is trickle charge the battery, just like is being done to the vehicle battery.
Useful amphours is the Reserve Capacity (in minutes) divided by 2.4. This rule of thumb was developed over many years in cruising sailboats without generators and is remarkably accurate in our campers if conservative.
The most important piece of info on the battery is the RC, not the amp hours.
With the refrigerator anti-condense heater on the 12v overhead is just over 19 amphours a day to power the hardwired LP detector, radio memory, and the little light on the water pump. That's a third of useful battery capacity before you turn on the first light or open a water tap. Without the anti-condense it drops to a more manageable 7 amphours.
Fully charged battery (24 hours) to start the weekend. Electrolyte topped off. All extraneous power turned off. The trailer overhead power drain starts as soon as you disconnect the charge unless you turn the battery off. Put a switch on the negative pole.
-- Chuck
