If you buy a park that is vacant, there is an underlying reason on why it is really vacant...it could be as simple as the owner literally turns people away and says no...normally not the reason
A few vacancies normally is the reason for this, not a totally empty park
It is either location, terrible park(crime, drugs, etc), bad management, failing utilities(private water/sewer) and some other reasons why as well.
It can be worth it to fill in the park, if you have the capital and the demand is there. Do extensive diligence on this before you move forward, also make sure the park is making some money,
Questions to ask, how do you plan on filling the vacant lots...organic moves in are likely, but you will have to pay for the tenants move and I don't recommend stealing from other parks as this could not farewell if your trying to fill in your park.
If you plan on using the Cash program...there a few caveat's to this method and you will want to do your research on those...it is a great program and is helping the industry a good amount as well
Filling w/ repo and used homes takes a lot of time and is harder to do at the moment because homes are getting harder to find in certain markets.
new homes can be very capital intensive and your capital could earn higher returns placed elsewhere.
Mainly parks w/ high vacancies(greater than 50%) normally are priced w/ potential like you said therefore are harder to infill.
Some people put $0 on a vacant pad, but other puts at least some portion to it in order to get to the number works.
In essense that vacant pad will cost you money because you have to mow, prep for site move in, etc.