Someone can correct any misinformation here, but my handheld does not relay through the truck radio. I use it out of the truck and it works great for the group in the immediate vicinity, but I can't really transmit beyond line of sight. I don't think analog relay works well in our use case because the same signal would bounce between all radios repeatedly.
If the industry changed so it was the default configuration or if you have someone VERY familiar with modern radios in your group (and everyone has a supported radio), I believe you can have everyone in the group switch to fully digital signals and enable automatic relay. When the signal is digital, the radios can tag each message with an ID and the radio can keep track of whether it has broadcast that message ID. Then every truck in line could easily and clearly communicate with every other truck, front to back. In this situation, your handheld could relay through any near radio. All that said, I've never been with a group that uses private, digital signals so this is all theory and pointless.