You will get sparking at the battery connections anytime you connect under a load whether power is incoming (from the converter's battery charger) or outgoing (to your clock radio, LP gas/ CO detector). Make sure the polarity is correct, put on your safety glasses and connect away! You can reduce or eliminate sparking by turning off your shore power and pulling either the in-line main fuse or all the individual ones on the 12 volt circuits.
Be advised that charging a lead acid battery produces hydrogen gas and a spark at the terminals can cause a small explosion with enough force to blow the top off your battery and splash sulfuric acid on you!!
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.