The goal around here is to have manager do as little as possible. So with that -
Why do you need invoicing? Reminders are nice but a waste of time when you're not submetering anything. Several Parks have their entry sign say "rent due by the 5th of the month" if it's really that important. Tenants talk and if you've implemented the "no pay no stay" rule then invoicing is pointless.
Does the on site "manager" do a good job mowing? I have seen them do typically a below average job which might be okay, but I would rather hire out the work to someone who knows what they're doing - in the end it will be less than the $3,000 per year you will be paying. These managers always break equipment and have excuses why mowing cannot happen regularly, especially when you're 850 miles away and don't know it anyway.
The only management needed is fielding any repair calls, enforcing park rules with a monthly check (or with an enforcement program using the tenants as your eyes and ears - e.g. cash for compliance), having gravel delivered a few times per year, and filling your last pad. Estimate the time for these activities and pay accordingly. Sounds like 100 bucks per month, and your on-site manager should start paying lot rent.
Others have different ways they would do it, that's just me. Good luck.