Hi Dan,
There are no specific service internals for this Webasto unit, however a good sign that a service would be advantageous would be failed start up attempts or a heavily smoking exhaust system.
I echo the advice given that the 'combustion air' and 'fresh air' are separate, although I would suggest that a failed gasket is unlikely (however getting this checked is never a bad idea). The separation between the 'fresh' and 'combustion' air could/will be compromised if your 'fresh air' inlet is sited next to the combustion air inlet rather than having a specific air intake from the cabin, outside for example.
If the combustion air pipe is 'leaking' exhaust fumes back down the combustion air intake pipe on shut down, it is possible that the fresh air inlet is sucking these fumes in. In any event I would strongly recommend you (or someone) checking the location of your intake pipes and ensuring that no combustion air can mistakenly be distributed around your boat.
I hope waffle this helps a little!
Tom