I have done loads of artic trucks and tankers over the years. Many fully wrapped with others part wrapped or just graphics.
To get the level you want to find the chassis line and measure up from it. make a mark and move along, and repeat.
Then, use 50mm masking tape. run the tape along the marks in one continuous line approximately the length your graphic will be.
What this will do is.
1, Allow you to stand right back and physically see if you are happy with the level.
2, It will also let you see if the line will run into any obstacles. (see the attached GAP Tanker schoolboy error)
this graphic is running straight, but the supports at the front come around an extra 50mm or so higher than the ones at the back.
You want to run the graphics at the same angle as the tank. running them level will not work. hence why running with the chassis line is the best bet.
Sometimes you cannot get access to the chassis line to measure from because pipes or cases are mounted to the side. however, these pipes and cases are often run from the chassis line too. so you can measure up from them in the same fashion.
You are correct about your process.
Tape the graphics completely into place and work from the middle up using a centre hinge.
then repeat by taking the lining paper out and working your way down. as shown in the attached illustration.
Where “you will” struggle. is because you are working alone.
I would normally have an apprentice holding back the flap of vinyl because as you squeegee the top section down, gravity will want the print to flap down onto the top of the take.
The times of print i use are only 52inches wide, yours will be 3 metres wide, but much shorter. still, this part will be awkward. so try and allow for this happening.