Yeah take a tent... I have a very small tent which I only put up in organised campsites to keep my gear in (I sleep outside of the tent if it isn't raining anyway). Many campsites in europe won't let you in if you don't have a tent (or maybe hammock).
It can rain even in the summer and when there are lots of mosquitos it can feel great to crawl into a tent (btw, euro-mozzies don't have infectious diseases, (malaria, dengue...)

If you can lock your tent zip with a small padlock your gear will be a bit safer (theft iis rare from tents but certainly can and has happened, don't leave passport and money unattended), and lock your bike, but don't sweat the security thang, it ain't a big problem...

Don't worry about the weight, your leg's will get strong!