You can get into Zancuda in SP with no wanted level and roam to your heart's content.
As Trevor, begin a hunting mission. When you've used your call and the red 'hunting area' is highlighted on the map, just walk away, get in a vehicle, drive to Zancuda and because you're in a mission you'll find that nobody pays you any attention there.
In order to steal a jet, helicopter or tank - do so but go back to where the deer are in your hunting mission. Fire rockets at the red hunting area. This will startle the deer and end the mission. Press B to exit - don't retry - and you'll be left with your vehicle to go and save it.
While you're there you can get a mini gun from the top floor of the tower too.
Note: If you kill anybody while on the hunting mission it will end. In Zancuda that will leave you at the fort with the usual wanted level.