You are quite correct. You cannot connect your iPad to your truck (or anything else) using this arrangement…
Your fifth generation iPad Pro has a USB-C port.
You cannot use Lightning accessories with this port - as Lightning and USB-C are entirely different digital interfaces. As such, while you can successfully use the Lightning to Digital AV Adapter with your iPhone (with a Lightning port), the Lightning to Digital AV Adapter is not compatible with your iPad Pro’s USB-C port.
To connect your iPad Pro via HDMI, you will need a USB-C to HDMI adapter - such as the Apple USB-C Digital AV Multiport Adapter:
https://store.apple.com/uk/xc/product/MUF82ZM/A
There are many available third-party USB-C to HDMI adapters - but very few support HDCP. Apple’s USB-C Digital AV Multiport Adapter and Lightning to Digital AV Adapter support HDCP - this being essential if you are streaming copyright-protected media from your iPad/ipHone to an HDMI monitor/TV.