Also note: When hooked to the sewer do not leave the black tank valve open. Ideally don't empty the black tank until its at least 2/3 full. That gives a good flushing action to get the solids out. (You can leave the grey tank's valve open when connected to the sewer, however.)
Look in the cabinet under the sink. Behind the drain pipes you'll see a pipe sticking up that just sort of stops. At the top of that is an air admittance valve. Grab it and see if it is loose (if it is tighten it up). The valve is supposed to only let air into the black tank but if it is loose it could be letting the stink into the bathroom (your bathroom sink also goes to the black tank so make sure the trap has water in it as well).
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