I’ve never really had this but off the top of my head I know Biotene mouth rinse and/or gum works great for dry mouth.
I’d try gargling with warm salt water. Gargling with peroxide but don’t swallow peroxide.. or diluted ACV… still just guessing… so if nothing else it will burn the bajijis off of you!! Not funny so here’s what the research says:

https://www.phdental.com/oral-health-news/2019/....

https://www.hss.edu/conditions_oral-concerns-lu....

https://familymedicineaustin.com/tips-to-treat-...

Lupus Mouth Sores and Treatment for Oral Health

https://www.phdental.com/oral-health-news/2019/may/lupus-mouth-sores-and-treatment-for-oral-health/

What having lupus mouth sores feels like and how they can be treated

https://familymedicineaustin.com/tips-to-treat-lupus-mouth-sores/

Lupus and Mouth Sores, Oral Complications | HSS

https://www.hss.edu/conditions_oral-concerns-lupus.asp