Welche Maße darf ein LKFZ (Leichtkraftfahrzeug) haben?

You can find the measurements in EU Directive 168/2013:

All vehicles of category L

(1) length ≤ 4 000 mm or ≤ 3 000 mm for an L6e-B vehicle or ≤ 3 700 mm for an L7e-C vehicle; and

(2) Width: ≤ 2 000 mm, or ≤ 1 000 mm for an L1e vehicle, or ≤ 1 500 mm for an L6e-B or an L7e-C vehicle; and

(3) Height ≤ 2 500 mm and

You can drive all subspecies L6e.

L6e-A falls out here as a motorhome is not a quad, so the measurements apply to L6e-B: length below 3 meters, width below 1.5m and height below 2.5m. It should be large enough for a camping chair.