As long as he is capable of performing all the pillars and conditions of the salat such as the rukoo and the standing and the sujood and the facing of the qiblah then it is allowed to pray in the truck. This is noticeable from the Fatwa of Shaykh Abdul Azeez Aal Ash-Shaykh in a video of his titled “The ruling on praying in the car during a necessity”. There then it will become similar to the ruling of praying on an elevated bed.
Ash-Shaykh Saalih al-Fawzaan, may Allah preserve him, was asked the following question:
هل يجوز أداء الصلاة على مكان مرتفع عن الأرض كالسرير أو نحوه
Is it permissible to perform the salah in a place that is elevated from the ground such as a bed or similar to it…?
لا بأس أن يصلي الإنسان على شيء مرتفع كالسرير أو نحوه إذا كان طاهراً وكان ثابتاً لا يحصل منه اهتزاز وخلل على المصلي وتشويش على المصلي.
“There is no harm for a person to pray upon something elevated [above the ground] such as a bed or its likes as long as it is pure and stable whereby it doesn’t become shaky and harmful and distracting for the one praying ”
[al-Muntaqa minal Fataawa (2/143)]
AbdulFattaah Bin Uthman