I have been to Morocco 8 times for events in Marrakesh. Didn’t go last year so my info is not totally up to date.
You need agents. These do not come cheap. We have paid various prices but they all return the same level of service.
Arrive in Algericas and park up at the Isla Verde area. You will need an agent to help with your carnet and they will book your ferry ticket once this is completed. No customs, no ferry ticket! We used Partida Aduanas in Algeciras. Ferries don’t have a strict timetable but I recommend the 0600 departure. You must visit the Police on the ferry and they will stamp your passport to say you are clear to enter Morocco and the port. You normally get a meal ticket for use on the ferry.
Once you disembark in the Tangier Med Port, you will need the help of another agent. We used Meyer and Meyer. Agents aren’t allowed in the ferry area so you have to park up on arrival, catch a shuttle bus up to the customs area, meet your agent, wait for customs processing, return back to your truck when your agent says so, wait until you are called by your agent and then you can drive up to the x-ray scanner. The port is relatively new but it is an absolute mess. Abandoned vehicles, trailers, poor signage. As you make your way to the scanner area, you are directed to a parking area. You wait here until your agent arranges a scanner pass and then you proceed towards the scanner. Then join a disorganized queue and make the slow journey to the scanner. And it is slow. Arrive at scanner, get x-rayed, and when it’s all completed, park close to the scanner… and wait some more. You will be relying on your agent to ensure customs is cleared and then you will be issued with exit paperwork and you are free to leave the port. This whole process will take most of the day so allow a short drive after the port to get to a petrol station and wait till morning. Plenty of petrol stations, very cheap fuel but not European standard service areas.
Services have been safe enough for us. Tolls on the “motorways” were payable in local currency only so you must get this when you are waiting in the port. Use your agent. Tolls to Marrakesh cost approx £45 one way. Road signs are in English on the motorways. Locals run on highbeam at night so prepare to be dazzled. Slow down at the tolls as cops are present and keen to get “tea” money for speed infringements.
Returning to Tangiers Med, you head into the pre port parking area and await your agent who will help again with your paperwork and entry to the port. More waiting around will happen and finally you will later arrive back at the carnage that is the scanner area. Same procedures as before. Your agent needs to take you to the Police so you can get your passport stamped. Do not forget to do this.As and when you are finally free to go down to the ferry area, ensure your agent has given you ALL the relevant paperwork and permissions. Your agent cannot go the ferry area so you are on your own. You have to pass several barcode checks and also customs police and make sure they do not keep hold of your paperwork as that will be the last you see of it. When you go to drive on the ferry, there are more police and you will be asked for it. Any discrepancies, you will have to return to the start. 
I have fallen victim to this one.
You will have been in the port again for most of the day.
Try to memorize your drive to the scanner when you first arrive as the signage in the port is sketchy and it will help you on the return trip.
Your agent should have told you, or phoned you, to say which ferry company you are with and your approx sailing. There are a couple of different companies running the ferries. Park somewhere you can see the boats. It is very disorganized. Don’t just sit in your cab when a boat is loading, go and check to see if it is yours. They will not come and find you.
Hopefully you will soon be on your way back to Spain. Once you arrive in Spain, a quick passport check and then you have to park up in the Isla Verde area. You need to phone your agent and they will come and collect your paperwork for customs(the agents in Spain have mopeds) and fingers crossed, its all stamped and cleared and returned to you. On a good day, you can be heading out of Algeciras in a few hours…but factor in a wait.
Good luck.