Motorhome weight

Hi all, I want to go touring in Germany and Auustria , but as my motor home is showing its weight as 3650kg this put me over the limit for a car in Austria, and I'm not really sure how difficult and exspensive it will be. I've looked on several web sites and I understand that I need some kind of device fitted to the windscreen and have it pre topped up with around 75euros of credit...how easy is it to get this device, do I need any paper work with me , and is there any other way round it? Is it possible possible yo have my motor home weight derated , I've had it on a weigh bridge fully loaded and it comes in at 3300kg ..any ideas ?