Filtered means without any deviation from a pure 12 volt DC. No surges, no interference. Filters are electronic devices that absorb voltage variations. Direct connection to a dedicated battery would provide such a source, but of course eventually the battery would be depleted. Battery chargers, on the other hand are usually "unfilterd" they provide 12 volts (or more likely something close to 14 volts) but it is "transformed" from 120 volt AC and still has some of the frequency variations of the original AC source.
For practical purposes, the trailer's battery will absorb and overcome the charger's "noise", so a dedicated suppy from the trailer's battery should be OK, but if you were to use a simple transformer as a power source, it would be a problem because it would be "unfiltered". The little plug-in cube transformers that are often used as "battery replacers" to power small electronic devices are also "filtered" sources and you could probably use one of those if you can find one with right voltage and current rating.
All of that is a long-winded way of saying, "use a dedicated connection directly to the battery though a fuse". (You might be OK going through the existing fuse panel, but that is directly connected to the converter and might have some noise introduced from there, as well as from other 12volt devices that may be running).
Edited after further thought: A dedicated source may be the ideal, but it will not be all that easy to do. I think I would at least try tapping into the existing 12 volt lighting first to at least see if that works. Make sure the wire size will support the load though.
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