I'd start with reaching out to local real estate agents who perhaps have a client who is building and wants to be close while they build.  We don't "manage" any rentals, but we do have a few past clients that while they don't want to do short term will rent out for 6 months to a year for people who are building and it's always worked out great.

Park newsletter or website.

VRBO

AIR B&B

Vacasa.com

FurnishedFinder.com

Carol Williams I like Don Baker 's suggestion but all the above are good.

I would poll the neighbors who may have friends & relatives eager to take advantage of your offering Carol Williams Perhaps a leave-behind with a referral gifting? Yes to staying away from predatory Craigs list

As I read this, one such community in my area comes to mind. It is a 55+ gated community.  The Community has a newsletter and the management office has a list of rentals. Many will put the rental listing on the MLS as well. 

Beyond that, any online advertising these days is questionable, IMHO.

We cannot vet too much.  I wish your friend all the best with this. 

note card on the community bulletin board?  Local FB market place.  It could be interesting  Chances are that you are gonna kiss alot of frogs before you find a prince.  

In seasonal areas it is often more profitable to rent to seasonal tenants, many people seeking a mobile home on a full time basis can be some rough tenants.

Perhaps give your seasonal tenants an annual lease with most of the rent being paid during the cooler months, with the idea that they will have it secured with their belongings ready for the next season.

In my area, the parks themselves have websites where they advertise what's available. I'd totally avoid Craigslist too!

Why would friend not want to hire a property manager to find her a good tenant? That's what I would do.

In our MLS, we can list rentals of mobile homes that are in parks.   Probably would have to pay a bit for the sale.   I would contact a property manager too, as another has said - they would charge a one-time fee.

When in Florida my clients had the best success with a 3X5 notecard on the community bulletin board. Worked like a magnet. And that was good since I did not do rentals.