Maybe my trailer was set up differently, I could access my battery while the trailer was closed, since it sat right there in the open on the tongue.

The Guest chargers are quite good.  I bought a 2612 (if memory serves me) from West Marine.   About $100 bucks.    If you want, you can find Guest chargers that can be permanently installed.

Don't try pushing more than a 10amp charger, the wiring harness might not like much more current than that.

Two more things to consider.  First, the battery really should be ventilated when charging.  The vapors are nasty and potentially explosive.  Second, if the wire from the connector to the battery is long and a light weight guage, you might never really get a full charge on your battery as there could be a voltage drop over the wiring harness.