You’ll have to look over your timeshare contract to tell for sure as to the legality question.
That said, many timeshare companies do allow for special assessments to be billed to timeshare owners for a variety of upgrades and repairs. Maintenance fees do not cover special assessments in general. Some special assessments have been as high as $5000 this past year. But special assessments need to be paid to keep your account current with the timeshare company.
Try to contact your timeshare company and negotiate with them. that’s the best you can do.