I would start with assuming the expense would end up being around 40-45%. As smallish park like that would tend to have higher expense ratios than a large park. But you really need to build your own no-BS, pro forma numbers.
A low expense ratio from the seller could be lying or it could be deferred maintenance and it could be both.
There are not that many categories to figure out; utilities, insurance, taxes, repairs, reserves, yard, payroll, and lumping all the little stuff like, office, advertising and banking, together in a misc account for the purpose of your pro forma. You should be able to get pretty close to real numbers.
If I was doing it I would not count POH income and would plan on selling them to the tenants if there are any in the deal. The problem with counting POH income is that the big expenses do not occur every month or even every year. You could be going along just fine and then have several move-outs that wipe out all of your POH profits.