The only sure way to 100% prevent scale and odor is to not use the head
If your head is salt water fed, then flushing it just runs dirty sea water into the holding tank. There are chemicals you can add that will reduce odor but over time things will need maintenance.
If you have a fresh water vacuflush type of head, you have a much better chance of less odor and less scale buildup.
FYI, if you have opened the seacock for raw water (assuming raw water operation) and pressed the button or pumped the unit to see if the bowl fills, pee or no pee you have already used the head!
On a trailer most heads can be flushed as far as the bowl goes. Just use a hose to fill with fresh water and then 'flush' into the holding tank. To flush the holding tank assuming you have overboard discharge, you need to place a container under the discharge port, open the seacock and run the macerator. Then you have to legally empty the container. Depending on what was in your holding tank, that task can be messy and smelly.