Your toilet is a Thetford Aqua-Magic V Hand Flush, part/serial number 31683.
https://www.thetford.com/product/aqu...-v-hand-flush/
https://www.thetford.com/part/aqua-magic-v-hand-flush/
Number 8 in your manual is the Water Module/Water Valve, part # 31705 Water Module Assembly. It doesn't look like there is a screen built in nor is it serviceable. If there is a screen inside your only option is to replace the water module.
It does look as the linkage can wear to the point that the valve doesn't open completely, part # 36528 Water Module Link.