Noticeable sparks and a drained chassis battery (coach chassis or car?) sounds more like a short to me. Check the ground wire to the trailer by all means, but you don't usually get big sparks and heat if the trailer is grounding through the safety chain. hat suggests that a LOT of current is flowing through it. Maybe you would have that symptom if all the trailer lights were on (tail, markers, brakes, etc) but you don't usually see that just when hooking up.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL