The system should certainly be cleansed to ensure the safety of the safety of your health.
If you have a filter in the system it should be removed and n a new filter fitted AFTER you have sterilised the system.
Milton is safe to use as long as you follow the makers instructions on strength and time left in the system.
Having spoken to Truma who make the majority of water heaters fitted to vans now, they agree that Milton is ok as long as it is not over strength and not left in the system longer than recommended (2 hours from memory).
Flush thoroughly after sterilising , fit new filter if one is fitted and enjoy the season.
If you use the Aquaroll to supply the water for the sterilising process, you have sterilised that at the same time!