If we look at the odors of dead animals as another kind of "pest" we recognize that the best approach if it is possible is to find the source and eliminate it. We can collect foul odors and we can mask them with other odors, but as long as that decaying rodent is still present and decaying it is going to stink, and even after it has somewhat dried it will continue to give off the awful smells. This could take some time to complete, so if you can make a careful search using your nose as a guide perhaps you can locate the dead animals, carefully place them within sealed plastic bags, and dispose of them. Doing this enables you, then, to thoroughly cleanse that area to remove any remaining traces of the rodent, which also could continue to create smells and attract flies. There may also be a need to treat that location with a contact insecticide to ensure any wandering parasites from the rodent are killed, such as mites and fleas. The use of an anti-bacterial disinfectant may be appropriate for cleaning the surfaces the rodent rested on, or you may even want to gather and toss loose material there such as insulation. 

If you cannot locate the offending source itself then you are stuck with trying to treat the odors from a distance. If you know it is within a wall but the customer does not want the wall damaged by opening it then you may be able to use a void injector fogger to blow a deodorant mist into the voids. Univar carries a number of effective liquid deodorizers that do not just mask the odor with a stronger one, but which are microbial in nature and actually feed on the molecules that are creating the smell. These may be products such as Bac-A-Zap, Rat Sorb, or Odor Hunter. Some of these come in small spray bottles for use on the surface itself where that dead rodent was lying, and are always more effective when applied directly to the surface. 

I also have received good feedback from PMP's who have used the dry deodorizers, such as the Odor Remover Bags and Pouches. These can be placed or hung in the area where the odor is strongest and according to the manufacturers they will remove the odor from the air itself. It sounds magical but the results have been very good.