I would take a look on the roof to see if there are any visible cracks. Then I would buy a can of that black tar for roofs and slap it on any possible leak areas or seems. I was able to repair some leaks I had on a previous truck camper this way. As far as piston slap I would try an oil additive such as dura lube, motor-up, or slick 50. I have used all of these products on previous vehicles. I put dura lube and motor-up in my Toyota this summer. On our 6,000 mile vacation this summer I only used 2 quarts of oil, and that was during the last 2,000 miles.
Nate
