I'm buying round trip RER tickets to Chateau Versailles and my journey begins from Gare du Nord. On my return trip from Versailles I would like to get out at Champ de Mars? Is this acceptable since Champ de Mars was not where my trip began?
The RER tickets you buy will be fine for getting out at Champ de Mars even though you departed from a different station. The RER stations inside central Paris / Zone 1, are all the same zone/price, so it won't matter where you end your trip inside Paris Zone 1.
Side note: Round trip RER tickets will be two one-way tickets you can use in either direction.