A dead battery has nothing to do with the muffler. You could have bad connections, corroded or loose. Or it could be a bad battery, or voltage regulator, or bad alternator. Or it could be that you are running something with the engine off for an extended time. Radio, lights and so on drain the battery, and you have to run the engine for a while to keep the battery charged up.
Start by checking the connections, if you have tools, take off the negative post, then the positive post. The negative post will usually have a black cable, the positive a red cable. Do not pry up on the cables, do not hit the cables or posts, twist the connectors back and forth to loosen, or use a battery puller to remove them.
Then clean the posts and cable connectors with a wire brush, wash with a mixture of baking soda and water, the rinse with clean water. Be careful, try not to get anyof the corrosion on your hands, it is part acid. It probably won't burn, but it is poisonous and smells and tastes terrible. Wash your hands when you are done. Check the cables to make sure they are good, then put the positive on nice and tight, then the negative. Do not hammer on the connectors. The posts are lead, and they connect to lead plates, and the connections break easily. Once broken the battery is no good.
Not check the other ends of the cables, make sure they are clean and tight. If you have access to a volt meter, and someone who can show you how to use it, or will do this for you, read the voltage on the battery while the van is running. It should be about 13 volts, or higher, at idle, and when you rev the engine up a bit it should go up
to about 14 volts. If the voltage doesn't go up very much, or if you hear a squealing then check the belt for the alternator. if it still doesn't go up, or it is reading low, then either voltage regulator, or your alternator is bad.
Good luck.