Yes as MFL posted. The batteries will have an abnormally high voltage right after you take them off the charger. But that is what is called a surface charge. It has no real substance and will deplete quickly. So you either have to let them sit with no load or charge for a few hours to dissipate the charge before taking a reading. Or apply a decent load to pull the surface charge off.
With a car the recommendation is to turn on the headlights with the engine off for one to two minutes to dissipate the surface charge. Then turn off the lights and you can take your voltage readings. Since the trailer doesn't have headlights and most interior lights are LED's the furnace is the easiest way to put a decent load on the batteries.
Another way to look at this is picture your battery as a tank of water with the water level indicating the state of charge. Now blow a big soap bubble on top of the water (this is your surface charge). It will suddenly look like there is a lot more charge in the tank. But of course the bubble has no substance so if you touch that bubble it will pop and leave you with the true level in the tank.
