For larger areas which it SOUNDS like you have, you can purchase EPDM material from many commercial roofing companies and create a patch glued to the old roof material and caulked at the edges or eternabonded instead of caulking (your call). I would recommend that the patch overlap the edge of the part that is damaged by six or eight inches.
Good luck / Skip
2011 F-150 HD Ecoboost 3.5 V6. 2550 payload, 17,100 GCVWR -
2004 F-150 HD (Traded after 80,000 towing miles)
2007 Rockwood 8314SS 34' travel trailer
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