The trucks are parked on the ramps because truckers have experience and a communication system and they know the rest area is full. 90% of the trucks you see,even if over the road haulers are traveling a route that they do once or twice a week.
Back in the day I would pick up roll aluminum in northern NY and deliver to Ga. where I would load granite curbstone back to NY. Twice a week,sometimes three if I could make the hours work.
I had a personal relationship with every expansion joint in the road, I knew where and when to park to avoid DOT,crowded rest areas and the heaviest traffic volume.
I don't park the tt on ramps,#1 because it is illegal,while a trucker probably will get away with it,because DOT understand HOS requirements, a tourist is not under any such restriction.
#2 although few,more than one trucker out there is a real antisocial psycho and takes it extremely personally when an RV is taking up the last spot in the rest area. Talk to any professional driver and he will scare the pants of you with the number of people driving truck that shouldn't even be walking around free,let alone mixing with polite society.
Whether you know it or not,we are the most universally hated vehicles on the interstate. Most truck drivers have at least a bit of skill in maneuvering a large vehicle while rv travelers run the gamut of absolutely 0 experience to capable drivers,and therefore we are a wild card in most drivers eyes.
I always see advice to just do what the truckers do, my advice is just the opposite,stay the heck as far away from them as you can!