I have a 2013 C-300 and the tires constantly lose pressure. No punctures, dealer says the rims are fine, TPMS is reading accurately and this is my 3rd set of tires in 4 years. Any one know what is causing this? It's maddening having to stop and fill the tires 2-3 times a week and it's not just in the cold weather either. Looking for suggestions.

 

Obviously a leak somewhere. I'd try another tire place, Discount Tire or something for a second opinion. You're right in that it doesn't seem normal. Dealers are not to be trusted.

 

I just did this for my winter wheels/tires last weekend as two were losing pressure and they are not even mounted on the car yet.

Add a tbsp of dish soap (Dawn if you care for the env) to 1L/1Qt of water, mix it well, add liquid to a spray bottle.

If possible, lay (the tire) flat. I would take each wheel off if I were in your circumstances and do this with the wheels on a flat surface.
Spray along the edge of the rim where it meets the tire. You don't need to soak the entire tire, just the edge where the rim and tire meet and for the bead/seal. Any air coming out will cause the soap to bubble up. suds would start forming rather quickly. You would notice bubbles in about a minute, probably even sooner.
Mark the area(s) with chalk / take pics
Turn tire over, repeat step with spray along the edge of rim where it meets the tire.
Mark areas again / take pics

Valve stem - This can be the culprit for leaks. If it is the core, it's an easy replacement. If it is the stem, it will require breaking the bead seal at least on one side, but I would take it to a tire shop of your choice.

Remove the cap
Spray soapy water in the stem where the valve core is. any air leak will cause the soap to bubble up. Take pics if you see suds.
Spray at the base of the valve stem where it is coming through the rim. same as above, any air leaks will cause the soap to bubble up. Take pics of the suds.

If you don't see any bubbles forming, then perform the spraying along the remaining dry areas of the tires, including the thread.

Repeat for each wheel.

Optional (I thought about it, but did not do this part): you can track the time it will take you to do all of this for the tires which are causing you grief. prepare invoice with time spent and your hourly rate (estimate your hourly rate liberally but fairly), materials (water, bottle, soap). If desired, add Miscellaneous items: water and soap/detergent used to wash hands after procedure; alcohol wipes used to clean smart phone from dirty fingerprints, shoe sole wear and tear, pant squat wear and tear, laundry expenses (detailed electric, detergent, water, softener cost). Sub total.
Don't forget to add sales tax. Total the amount.
Bring the pics and invoice clearly showing the bubbling suds to the shop. Ask for Accounts Payable. Drop invoice off.

Edit: If you google the process, it will take you less time to see how it's done than read my post.

 

I take my suspect wheels and drop them in the horse water trough, and look for bubbles. Then, I have to refill the trough so it doesn't get complaints of tasting like rubber water.