I have a 2005 C4500 Top Kick Monroe conversion that was purchased new in 2/07. It is primarily used as a toy hauler for a 40' fifth wheel trailer and only used on weekends. I have had no problems with it until last October / November when the batteries would die after sitting for a week. I took it to the closest dealer (about 30 miles away) three times. Once they replaced the battery isolator (about $900) and the other two times they checked the batteries after driving to the shop and said they were both fine. Of course they would be after charging during a 45 minute drive. I called GM and they set me up with another dealer. They had it for a week and determined there was a draw on the batteries while sitting and causing them to discharge. They thought they narrowed it down to a DVD player but would not work on it since it was not "GM OEM equipment". It is funny that the DVD player did not cause any problems in the first 7 months of operation. Just for the heck of it, I removed it and disconnected the radio and CB. The truck sat for a few days and then would not start again. I called GM again and they set me up with another dealer to go to in a couple of weeks. Any one have any solution to this other than leaving a trickle charge plugged into the truck all the time.

Use a multimeter and backprobe all the fuses. Something is running when it should not be

Charging the batteries will have no effect on the battery test, IF they use an electronic battery tester. For piece if mind, have them checked with an electronic tester at most auto parts stores. Around here, Napa, Advance, and Autozone, all have the electronic (capacitance) type tester. A bad cell will cause your problem, also, yet the truck will charge fine.
I had the same problem with a wheel loader. Started every day, unless it sat all weekend. One battery tested bad (shorted cell). Changed the POS Interstate battery out, problem gone.

That is the ticket right there! One of the fuses will so a small amount of volts and that has to be draining the battery! My escalade did that last year it was about 2 weeks old and we went on vacation. When we came back the lot guy at the airport parking said man sweet escalade but it wont start! On our way home I dropped it at the caddy dealer and they gave me the same run around as you are getting! Long story short it was the window motor on driver side for some reason it would keep trying to go up but didnt make any sound!

Good luck on your problem! Keep us posted

There's been a few with air compressors [seat/exh] shorting out caused by the high pressure switch then shorts the relay causing it to run when off
But, you should be able to hear it, if parked locally

Try pulling the compr fuse and see if it still happens

FYI many of the "OEM Monroe" accessories are wired hot, very stupid. Exsample, the heated seats, the DVD player you mentioned, rear seat TV etc. All of them are wired to be constantly on, I'm working on a a few relays so I can rewire them to be triggered off igntion to prevent dead batteries due to small continious current draw. At least they wired the air ride compressors to be ignition triggered or I think we'd all have dead batteries:eek: My dump valve is always going bad and leaking, after a day of sitting my rear is on the ground:o: