No way to really put a trip on hold. You can add the truck stop as a Via or turn the recalculation to Manual (Preferences/General/route). This will basically keep the same routing when you pull off to an unplanned stop in your route. Note - turning the unit off and on again, it may pick a different route that the one you were on.
You can normally build a trip and if you add via points, go to Choose destination/Multi-Stop trips/Current trip and do a Save As and save it to another folder. Then when you turn the unit off/on, go back to the same screen and pick your trip folder. Do a Start Trip from this folder and then modify the current trip folder for the locations you do not need anymore.