The microwave element of a dualtec is ajustable in most if not all detectors i've seen but I'm sure their minimum range is still some 8 metres although this will be less if the detectors mounting height is reduced. I would have thought still too long for your motorhome. The microwave element also has a wide spread that would reach and break through the walls of the motorhome within a couple of meters. To have a dualtec fitted would mean a hell of a lot of adjusting including screening the sides of the lenses. By the time you are finished, it would be close to useless. A fibreglass body would not stop the microwave element detecting outside the physical perimiter of your motorhome.
Quads could be viable but you'd be defeating the object of installing one adjusting the pulse count to 3. Crow do a great quad, the SRP700 which if requested, can be fitted with a black lens which helps in those direct sunlight situations. The Crow Genius is another quad that is great in harsh environments. I've fitted one in one of those BandQ steel kit sheds with no problems. If no expense spared, the Crow D&D works fine in swimming pool areas and outside and works from 9vdc too.
Just out of interest. How is your alarm powered?
Tony