Since you have a rubber roof, just push! The roof is slightly flexible in the areas between the rafters, so just keep pushing until it gets hard. Don't push too hard though, as it might damage the roof. I guess a stud finder would work too, if you have one.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR