Bob? Forgive me, but from your post it SOUNDS like you intentionally connected the batteries in series to create a 24V power supply in the trailer.
A 12V battery at full charge will normally deliver around 12.7V, so, if you connect them in series, you're going to get about 25.4V (which is very close to the voltage you measured). You did nothing wrong, in my opinion, if that was the effect you were after.
But, did you replace all devices, lights, etc., in your trailer with 24V equivalents? The wiring shouldn't be a problem, since 12V wiring is heavier than 24V wiring. But, your water pump, lights, electric brakes, and so on won't like the increased voltage. Are you possibly towing the trailer with a vehicle that supplies 24V charging voltage?
As kwdstalker said, the usual setup is to connect the batteries in parallel - + to + and - to -.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew