The question of warranty implications and third party installers is often raised when fitting devices to our vans either DIY or 3rd parties.   Solar Panels, TV dishes, WIFI booster gadgets, replacing a roof light your self , to name a few.
 Refer back to your terms and condition as it is possible such a condition could be stipulated, but not enforced.
In reality manufacturers are well aware that owners will wish to customise their motorhome with various accessories however  the factory won’t cover any remedial action that is caused by a third party however the rest of the warranty will remain valid.
If for example you had an awning fitted by a third part and a leak occurred from a fixing point you would need to refer back to the installer,  however a factory installed rooflight leaked you’d refer back to your selling dealer or Service Partner.
If your dealer was to submit a claim to Dethleffs it will only detail the fault at hand and what remedial action is required; if this was a leaking rooflight then they would have no need to mention or be asked if the owner has had any modifications performed.
The best you can do is ensure any installations are performed by a reputable company.
Remember Dethleffs record all the details of your van the day it left the Factory and any additions fitted by Dethleffs Dealers .
Again , it is similar to the question, Do I maintain the 6 year water ingress warranty , or not . Do I have installations carried out by approved Dethleffs trained staff or third party . Basically you have the choice , but be aware there may be consequences .