In my experience it's often how you react to such attempts that makes the difference. It's important that you don't get yourself into the victim's position by starting to stutter over "Can I sleep in your tent?", but rather reply with a clear "Well, that wouldn't seem appropriate, would it?".
Say it with a smile, but be serious enough to get the message across.

Self-confidence and the strength to stand up, when you see that somebody crosses a line, really makes an impression. Just always stay polite, so people are less likely to get aggressive over the rejection.

I guess that's easy to say as a guy, but as mentioned in other comments, we do have similar problems at times. It's not like I'm particularly strong (or at least I don't look it to other people), so I do have my little fears as well. I just think it's important to not let them guide you. Common sense is a good guide and it sounds like you are using a lot of that already.

All in all, I think it sounds like you are rather well prepared. Sleeping inside a camper would seem safer to me. On the other hand, as I said: If you wanna sleep in a tent, then don't let fear make you get a campervan.

Roll on!
Robin