sounds like paradise with bad wires!
no chance of lobbying the owner for some new wire?
4 to 6 volts is not that bad, but with a multi-meter it is hard to tell. a good old fashioned meter with a needle would show a much larger swing.
it would seem that you are doing everything you can to protect your equipment.
given the past history of the park it is only a matter of time before the cable feeding your trailer fails. then it will get fixed one way or another.
the question is, you're paying for a site, you shouldn't need a transformer just to run your a.c.
can you move to a part of the park that isn't having problems(if there is one)?
sorry to sound stern, but i have yet to run into a customer of my utility that would let me off the hook if they had bad voltage.
make them fix it if you can, sounds like you've got a nice spot.