I'm assuming we're talking about the general appearance of the homes and yards, and not situations where the tenant has 10 junk cars, 5 appliances and 3 pit bulls in the yard.
In general, the first step is to send the tenants a letter in which you describe your expectations -- that you want to make the park the best that it can be, and describe in detail what that means (homes painted, skirting, yard free of debris, etc.)
Next, you set the tone by putting up new signage, white vinyl fencing and landscaping (if appropriate) and make the common areas perfect. This will send the message that you're serious about the clean up concept.
The good tenants will follow your lead and clean up.
The bad tenants will do nothing, either because they are too lazy or can't afford the materials.
Those that are unbelievably bad (particularly those with large dogs that refuse to remove them) should just have their leases non-renewed. They can then choose to make the necessary changes or leave.
For everyone in-between, you should make a lot-by-lot list of what you want done. And don't be a perfectionist. Your list should be items like "fix missing skirting pieces", "paint home", "get aluminum foil out of windows" and "remove refrigerator in yard", not "plant seasonal color to match your shutters".
Send these folks a letter asking them to make these changes. Do it a couple times. Then tell them if they don't have them done by a certain date, you're going to do them and potentially bill it back to them.
When that deadline expires, take photos of everything wrong and do the work yourself (using contractors). Henry Ford assembly line it: do all the painting, then do all the skirting, etc. You can then either elect to send the bills to the customers or just eat it.
On those that move out, you can ultimately take their homes through abandonment and then fix them up and sell or rent them.
In this manner, you can get the park perfect in around 6 months, regardless of where it starts out.
One final note, don't evict anyone unless you have to. Don't be petty. Recognize that the former owner might have had no rules, and you can't be a perfectionist in such an environment. Be fair and listen to what the residents say. Most people WANT to live in a nice park. You are trying to be the catalyst to that happy conclusion, not the dictator trying to force it upon them. Work with them and be a positive force, not a negative one.