Parts such as these BSP thread to barbed tails help you adapt your van to suit your chosen Flat Out sullage hose - as BSP threaded fittings including sockets, reducers, bushes and elbows are readily available at your local hardware/plumbing/irrigation/RV-accessories store which is where we recommend you source your connection pieces.
You can often also access pieces like these offered here at such places. Since the 32mm tail isn't accessible everywhere, we have added it to our inventory. The 25mm is here for convenience because the elbow in that one is so handy.
If your van is not ready to go for Flat Out hoses, we suggest that you first take off the existing fitting on your van. Then go to your local specialist with that fitting, and a barbed one that fits your flat out hose (even just the joiner we provided you with) and ask them to make you something that starts like your existing RV fitting, contains an elbow, and finishs with a barbed tail. They will have a wall of parts to choose from and should be able to make an efficient combination such as is in the examples here.
If you add one of these tail pieces to your order, you make the job slightly easier to find the middle bits.
The purpose of sharing so many ideas is not to overwhelm you but instead to assure you that there is certainly more than one way to cook this chook, and you will be able to find a suitable solution once you know what is on your van. Also we have found it handy over time to know the names of the various parts, especially if you are thinking of purchasing the bits online. Valve Warehouse and Irrigation Store are some good places to look.
Please note that the BSP labelling not the thread size, if you put your ruler over a 1" fitting, you will find the inside pipe is 1" but the outerthread is much larger, and then the female version of this will be larger again. if there is a label anywhere on your van for what is connected, we recommend you find it, or download a BSP measurement guide.