Welcome to the forum
Your trailer should have a converter that charges the battery when plugged into shore power. It will either be the bottom half of the power distribution panel or a stand alone plugged into the back of the panel or an outlet nearby.
If it’s the bottom half, look for the 30 or 40 amp fuses off to the left of the 12V fuses. If it’s a stand alone then the two fuses will be on the back side of the converter. You should be getting 13+VDC when plugged in.
__________________
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.