We often eat lunch while driving. On my coach, I have a drawer in the center of the dash. I've pulled that open and set things in there, like a bag of chips. I'm guessing your gasser has a wooden tray and cup holder on the doghouse. You can make a heavy round object or even fill a large cup with plaster or something heavy and mount a plate to the top. This would give you a plate at a good height and within reach when placed in the cup holder.
Even though there are times when we want to get there in a hurry, before dark or to beat traffic, you should ALWAYS stop every few hours and walk around for a few minutes to stretch and keep the blood flowing (clots), especially as we get older. It's also a good way to get indigestion.
We look forward to stopping at nice rest stop or other area and having lunch and sometimes dinner. It breaks up the drive.