We had this problem with a previous van. Bought some wide tapered corks from eBay and pushed them into the waste outlets (after wrapping a little PTFE tape around the narrow end to aid removal). Filled the waste pipes / sinks with about 5 litres of puriclean (or VWP steriliser) solution & left for several hours while we went for a walk. Place a plastic bowl underneath the waste pipes for emptying & problem should be sorted. The amount of gunk which came out was quite surprising!
We still carry out this procedure just for peace of mind on our subsequent vans from time to time, as we remember how bad the smell had been.
Hi, the problem was the alde system. Apparently if the wrong amount of solution of antifreeze is used (too little) it causes a bacterial build up in the solution that smells like stale water. The system was flushed out and changed and smell has gone.