01 Silverado Battery Drain

I have a 01 Silverado 2500HD 6.0 litre and if I don't drive it for a few days (5-7days) the Battery goes Dead.. I have changed the Battery twice since I bought it.. and No luck.. The other thing is that at night when I'm Driving and getting ready to slow down ( push on the Brakes) the lights dim...

Anyone have this happen or have an idea what to check???

Alternator... i have a 02 it's very easy to change.. But make sure your positive connection is on good. Ipost a message on here how to waste 200 bucks because i didn't check my battery connection.