I'm considering purchasing a 4500 ram. I need to know what CM truck body will accomodate a 5th wheel hitch at the correct height. Anyonehave a truck set up this way?

 

I don't think there is a standard. I have had 6 different rams and have simply had to make minor adjustments on the hitch or head of the trailer to accommodate. Most hitches have some height adjustment and a lot of RV heads have some flexibility. I have two trucks now that are not the same height in the bed. Buy the truck you want and adjust the hitch accordingly. There are those few RV's though that make that impossible.