https://www.thetford.com/wp-content/upl ... del-80.pdf
This could be the manual for your toilet.
Your toilet could be the same as toilets I have experienced in a couple of different tour coaches . The ones I have used have a holding tank under the toilet that is emptied via a hose that is connected to a slide valve under the holding tank. That is accessed from an outside side locker. it uses the contents of the holding tank to flush with with via a electric pump. You need toilet chemicals in the holding tank, otherwise the flush water is brown and yukky after it has been used a few times. Not a nice design, in my opinion. A separate flush water tank would be much more preferable.
After emptying the holding tank, an amount of clean water needs to be poured in via the toilet bowl with a hose or bucket. Just enough so the flush pump can start flushing.
Well that is how the ones I have used work, your may be different. Hope this helps.
2014 Leisureline 7.0 metre Elite Caravan towed by 2017 Ford Ranger. 4 x 150 watt solar, two 40 amp SRNE MPPT solar controllers, 1800 watt PSW inverter, 200 amp/hr Lithium Battery, 180 fresh water, 180 grey water.