The least expensive out of pocket ways: bandit signs / sandwich boards out on the street (if those are allowed by city ordinance), put an ad on craigslist advertising your park, snap a pic of the entry, write in the sizes of the lots (in feet like "I have 20 - 80 foot by 40 foot lots that would hold up to a 28 foot double wide or double wide offset" (if that is within your city code setbacks), the parameters of the park as far as rent cost, any amenities your park has, city water or well, sewer or septic, playground, clubhouse, monthly events and drawings, stuff like that... make the marketing ad fresh and eye catching, put in proximity to local grocery, hospital, free way, bus depot, airport, schools, and re-post it once a week to keep it fresh.
Put in your contact information for your office, email and phone, the basics.
Put an ad on mh village as well - people look at the village and craigslist for finding a spot for a house, besides driving around... and make sure you can be found on google and make sure your entry has some kind of street frontage sign or no one will ever find you.
You don't have to undercut yourself to get people in, there just has to be a reason they want to live there... so make sure the place is clean, trash free, kept up, good curb appeal, if you have a manager on site they are professional as well as professionally dressed, and your current tenants are keeping up their homes and lots.
Did you talk to the big jobs coming into the area? The staffing manager would probably be greatful for ideas on where to let their new staff know where there is local available housing options.
I see the post was put up almost a year ago, what happened since?