Obtaining the screen disc for Germany

I know that a disc needs purchasing for Germany to do with the Eco emissions but how do you get them and know which disc you want. I have just been looking at my V5 for a 4.05T motorhome, I know it is a Euro 4 engine, but there are no figures at all in the emission section of the V5, I presume because it is taxed as a private HGV. So how do the Germans know my emissions to charge me the correct tax?

Regards, Roy