You are correct that the RV-type hose and bayonet fittings only comes in 3" diameter.
The way I see it, you have four choices.
1) You can T the greywater and black tank dump valve outlets s together and have the wastewater from both of them drain through a single 3" bayonet fitting so you can use the standard hoses. This is what Airstream and most other makers are doing on new RVs -- it's rare to see a greywater tank plumbed with a 3" drain. That won't work unless the drains are close together though.
2) You can adapt to a 3" bayonet and just use the regular hose. You're going to need it for the black tank, anyway. Either use the offset adapter in the photos upthread or a plain adapter, depending on what fits the best.
3) You can build a custom 1.5" drain system. You would probably want to use cam-lock fittings, which are widely available in all sizes both plastic an aluminum. They aren't expensive. (Here's one source: Aluminum Camlock Fittings, Polypro Camlock Fittings
). There are a variety of choices for hoses.
4) You can use a garden hose. The main problem with doing so is that if you are at a dump station it may take longer than you would like to empty the tank. You will also probably have occasional problems with clogging. Like many people, I adapt down from 3" bayonet to garden hose for greywater when in a site with full hookups, in many cases, because the collapsible 3" hoses don't hold up well.
I'd opt for choice 1 or 2.