Here are my symptoms:
Crossfire will start and then immediately die. When I try starting it again it will not start and it just keeps turning over. I’m getting no fault codes. The car has a new battery. It has both keys and will unlock remotely. Tried a different RCM module from an SLK but the part number ended in 205 rather than 305 . When I tried starting the Crossfire I got nothing and just the electric fan coming on. When I tried my Crossfire RCM in the SLK it had the same symptoms as my Crossfire. Deeply frustrated. Should I buy a new RCM? No one wants to work at it and the local Mercedes specialists said it’s not a Mercedes! I have had trouble free miles up until now. If the RCM would fix it then that would be great but I don’t want to buy it and then discover it could be the SKREEM or something else. Help please!

 

1) NO ONE should own a Crossfire and NOT have a spare RCM. They plug in without tools, they fail a LOT and they are hard to find locally at times.
So, Order one today, if it fixes it, fine, order another to keep as a spare. If not, you should have one anyway. YOU DO NOT WANT a "Crossfire" RCM, you want an RCM for a 2003 Mercedes SLK320.
The supplier's website will tell you that it won't fit a Crossfire ignore that statement.

2) Sorry, but your text is a bit confusing to read. Let's just look at this:
Crossfire will start and then immediately die. When I try starting it again it will not start and it just keeps turning over. I’m getting no fault codes. The car has a new battery.

What is the fuel pressure? Clearly, the RCM's ECR output is working, otherwise, the starter would not engage. But is the RCM's output to the fuel pump working? Is the fuel pump working?

I"d buy a cheap fuel pressure test kit and see what the fuel pressure is at the end of the rail, 58psi +/-4psi is expected. If you can not hear the fuel pump run for about 5 seconds when you turn the key on, the suspects are:
Bad Fuel pump relay on the RCM
Bad fuel pump
Bad filter/regulator
Bad fuel pickup in the tank

The EASIEST thing to check is the fuel pressure, it's like checking tire pressure, cept if you allow a leak you can set the car on fire - but do not be intimidated, just do not allow smoking within 50 feet of you at the time.
I have one of these - cheap but they do the job:

 

Thanks for the Advice. I can hear the fuel pump working but I suppose I would need to test it.Next on test on the list. You are suggesting though that even with the starter engaging there is still the high possibility it could be the RCM.
.

 

Not likely, I did not think your pump was running. The MAJOR RCM failures that cause a no start are:

No Engine control relay output - starter won't engage.
No fuel pump relay output - starter engages but fuel pump wont run.

I suppose you could have a 'open' on one of the various outputs that feed various items in the car - but can't say we've seen that.

STill, owning and driving a Crossfire without a spare RCM and spare crank position sensor seems like asking for trouble. I've had two RCMs fail in three cars in 14 years and one crank sensor fail in that time. If you are not going to break out the service manual and confirm all of the outputs of the RCM, I'd order an RCM. Given your last post, I doubt the RCM is at fault, but again, you should not be driving a car without a spare.

GET ONE, they are valuable and easy to change: I talked a lady (Ella in Gainseville, GA) thru changing the RCM on the phone who barely knew how to open the hood - and got her home, from 400 miles away.

 

In his post he says electric fan coming on, doesn't that mean the RCM is faulty?

 

How I read it was that it did that ONLY with the "other" RCM, but not the RCM the Crossfire in question came with.
But I admit I might have confused that.

But again, that's why I say we ALL should have a known-good or new, spare RCM with us at all times. I know that I sure do!

 

You can take your RCM apart and hold the fuel pump relay closed with a rubber band.
Somewhere there is a post by tighed1 that explains all of this. Use the search bar in the upper right hand corner.
Here's a video that might help