There a ground and one ground only. It's the true ground coming from the power cord when plugged in.
If you're taking about the negative 12 volt from the tow vehicle, there is NO direct connection from the tow to frame of the trailer. Both negative and positive 12 volts are run through wires. It will appear that the negative is connected to the frame, but it's only in one spot. The converter has the ground wire from the shore power connected to it chassis which is also connected to the negative 12 volts.
Go to the document center on this site and download the Scamp
manual. If you don't understand the drawing of electrical
system it might be best to contact somebody that will understand it.
12 volt systems are not repeat NOT as safe to mess with as you might expect. The only difference between 120 Volt systems and 12 Volt systems is you can get a nice little shock from 120 system. But as for fires the 12 system more likely to cause a fire because of the misunderstanding of the dangers thus a non knowledgeable person will mess with the 12 volt system and create problems the might not show up for quite a while.