Bourton rubber smell

I seem to recall mention of a burning rubber smell, perhaps coming from the heater. In my case, it appears not to be the heater, but comes from under the offside bed/seat. It also seems worse when the batteries are charging. (Two lithium batteries)

I have checked and neither the batteries, nor the wiring, or for that matter, anything else in the locker is hot, or even warm. The smell only appeared after a few weeks use of the van. It does not come from the boxed in section under the rear end of the locker. I have tried removing everything from the locker in order to isolate the cause, but with no luck.

Has anyone else experienced this, or have any idea what the cause might be?

The most likely cause is fibres and dust which accumulate on the heater element as air is drawn through. My wife complains of a smell from ours at the heater outlets under the wardrobe.