Here is the problem, if your posting of the message in hex is correct, the message can contain any hex number from x00 to xff and you cannot differentiate a STX or ETX from what is included in the text.
So, my solution would be to store the fixed count of bytes in an array until the count of bytes matches exactly the documented message length.
Then process the array, using only the data portion.
Then request the next message from the truck and do it all over again.
Paul