Gina, neither the hosts nor their ranger you're dealing with understand the spirit of the rule.
When I first took out insurance on the micronaut, I was surprised it was inexpensive.
My insurer explained that once it was hitched to the truck, they were considered one vehicle, so it was just a rider
on the truck policy because the "one" combined vehicle had greater value than the truck alone.
It is very specious to consider the travel trailer a separate vehicle once it is unhitched.
When unhitched, it has about the same "vehicle" characteristics as a tent, that is to say, none.
(Unless you forgot to chock your wheels!
If I were faced with mindless bureaucrats insisting on extra payment, I would have argued until
blue in the face
against their logic, no matter how petty the extra charge.
Using logic, you could have asked them, "How many vehicles did I arrive in?". Answer: One.
If you arrived in one vehicle, how could separating the two halves of the one vehicle possibly result in two vehicles?!
How could you have two vehicles present, when you arrived in one vehicle, have not departed and come back, and no one has joined you?
Separating the ONE articulated vehicle does not in any logic make it TWO vehicles, especially if one half is immovable under its own power.
Leaving your trailer behind for the day is, for occupation purposes, exactly the same as leaving a tent set up.
If I were up against this, I would leave the trailer, and on the second night, park the truck outside the park and WALK in. When they come for the fees,
point out that you only have HALF a vehicle and demand half off your already half-off discount!
That'd show 'em!
The idiots you were dealing with may have been nice and hospitable, but they were still idiots.
Nowhere I have been would anybody interpret "2 vehicles" to be anything but two motorcars.
On Edit: Obviously the SPIRIT of this rule is to prevent congregations of group campers and/or "party-ers" from overrunning the family campgrounds.