There are some out there with short time frames. Just keep your eyes open. I would check
WorkampingJobs.com - Free Job Listings from Campgrounds and RV Parks in the state you want to be in OR
www.volunteer.gov (assuming you may want to volunteer). Join Workamper news and get the emails every day with the new listings.
One thing that a lot of people ignore is just to cold call campgrounds. We have been fulltime for 11 years and I've always found jobs on my own. We choose the area we want to be in, then cold call rv parks in the area or check websites for state parks. If you can't find info on their websites I call or email the rangers. Sometimes parks lose hosts mid summer and are looking for short term people. There are other jobs that are short term like Pumpkin and Christmas tree lots, beet harvest in the northern states, wheat harvest in MT.