I visited the Royal Pacific Hotel at Universal last February - told the guard I was interested in just taking a look around and taking some pictures. He let me pass with no questions asked. Can't vouch for how it would be handled at the other two resorts.
If the resorts do charge for parking, I suspect it's added to the bill - the guard shacks don't look like they're equipped to handle money exchanges.
The main lot at Universal charged $8 per day five months ago, when visiting the parks - haven't heard of any increases.