If I remember correctly, in the top left hand corner of the screen you should see an icon of a car. Click on that & it will ask you if you want to change vehicles or will give you an option of auto or RV. Click on the RV & I think it will ask if you want to create a new profile, click on that & it should start leading you through the steps. It will help if you already have the measurements, length, width, height and weight. Keep in mind this is total for your tow vehicle, and rig. I had already had these handy before I started.
If this isn't correct,
here is a link to the owners manual on Garmin's website, which goes more in depth than what you get with the unit. If you still have problems PM me & I'll get mine out & go from there.