I wouldn't worry too much about it. It's a new "toy" and the kid was exploring. You see a ladder, you assume you can climb it. And you must be expected to be on the roof because otherwise why have a ladder go to it.
Explain why the ladder is there and why he shouldn't walk around on the roof. Then stop worrying unless he does it again. Most kids at age 8 can understand logic. Explaining to him that the ladder is there to be able to check the A/C, etc., but that the roof can't support his weight without being reinforced will likely be sufficient.
You could also try redirecting him. Show him things that he can help you with. Show him the underneath of the trailer - the various tanks, pipes, etc. He's got curiosity, direct it in ways that help you. How many folks here on this board learned to do things by "helping" dad or grandpa or mom or grandma when they were young. Start with him now (perfect age) and he'll be here posting about his new trailer in twenty-odd years. :)
As for others climbing your ladder, I wouldn't worry. Most folks won't go climbing it unless you've asked them for help with something.
If your grandson does climb it again, then you've got a couple choices:
1. Let his parents handle it; or
2. Handle it yourself in one of the following manners:
a. identify something he likes that's in your control and use that to keep him off the ladder (i.e., no camping trips with us if you climb up there again);
b. identify something he doesn't like that's in your control and use that to keep him off the ladder (i.e., if you climb up there again, you'll be mowing my lawn every week for the summer); or
c. block access to the ladder (strap your camp chairs to it, use wood as suggested above, etc.).
