Many years ago, a Doctor recommended soda crystals. These can be obtained from 'Wilko's' for a £1 a kilo bag. I put about a handfull into the wash basin, mix with hot water and drain, into waste tank. over 15 years of our van, never have any problems. Can even open the driving window and don't experience the 'Nif'smell being imorted into the driving area.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.