do i need to hire viatoll box or can i py cash / card on the toll roads, we are traveling from calais through dresden, if viatoll is a must where can i get one
It's obligatory to use ViaToll if your motorhome is more than 3.5t.
When you drive from Dresden, the last service area on the A4 is called "An Der Oder" - it's a few hundred metres before the border. Pull in there, and drive to the far end of the car park. It's quite hard to explain, but it's here - marked as "Toll Road Rest Stop" on Google Maps. When you first enter the service area, you won't see the ViaToll point at all - it's very badly signposted. Be careful - it's easy to miss the ViaToll point, so I'd suggest parking up next to the toilets and walking from there.
You need to get the ViaToll box there, as there are no physical toll booths between Gorlitz and Wrocław. After Wrocław, you use the viatoll lanes (far right) on entry and exit to the first tolled section between Wrocław and Gliwice.
After that, I'm not sure if you'll use the A4 between Mysłowice and Kraków, but if you do, it's cash/card only, not ViaToll.
There are plenty of sections of non-motorway roads that are also charged by ViaToll - you'll notice them by the presence of a small orange sign showing the ViaToll tag. Make sure your account is well topped up - there are plenty of checks by the Road Inspectorate for ViaToll violations.