Many trailers the converter is directly in back of teh fuse/breaker panels but you said yours is not there.
On my motor home it is nearly as far as you can get from the fuse/breaker box.
I have seen them.. In a bench seat (under the seat) storage area area, In basement storage compartments, and other places.
Recommendation is this Replace that automotive charger with a Progressive Dynamics 9200 sized to match your batteirres as follows.. And be done with it.
Group 24, about 75 amp hours each
Group 27,,, Think 95
Group 29 Like 1105
Group 31 130
GC-2 pairs in series 230
Add up your battereies, multiply by 30% (0.30) and.... Get the converter closest to the result.
Home is where I park it!