I have a 2001 c240 .. My problem is I can't let it sit for mre then 3 or 4 hours without it dying and needing a jump. I have replaced the battery twice.. I took it to mechanic n he tells me it's the fuel filter that's the problem n thatmy newly bougt battery is bad... What do you think??? I'm lost.

Has the new Battery been tested? has the charging system been tested? never assume anything, I got 3 defective batteries in a row once, used cheap discount auto parts store battery, never will make that mistake again. Kragen and Autozone parts stores will test the Battery and Alternator for free.
Another thing you need to have checked is are any interior lights staying on, go out at night and look, check the visor lights if equipped as well and the hood and trunk lights.