The problem is generally due to one of the following decomposing food caught in pipework or the ridged flexible plastic piping used for the drains. Milton is excellent although it is only a refined bleach for food use! Crawl under the van, put a cork in the outlets, fill slowly with cleaner and leave to stand. Repeat as necessary. Keep fingers crossed there are no leaks in the pipework!
I have also used crushed Steradent tablets - the foaming action works sometimes.