I've been in Marina de la Paz many times over the last 20 years, never needed secure parking though as we always leave boat and trailer in San Carlos, Sonora.
The parking spaces inside the gated area are limited especially with a trailer but they might be able to squeeze you in, Neil is very accommodating once you are actually there on the spot vs. trying to make reservations ahead of time. Same thing for slips, the only reservations he will take are for long term slip holders
There are several boatyards and small marinas adjacent to MLP that would be a good option for storing the trailer.
I agree that a secure storage is a must for peace of mind if nothing else.
Alain Provost/ Deja La