I don't know what Eternabond tape is made of, but I use it to fix leaks \ cracks in the roof of my RV and I recently put a coat of liquid EDPM on the roof of my RV, the company that sells the liquid EPDM says to sand the Eternabond tape and put a coat of contact cement over it before applying the liquid EPDM rubber. So I don't think it is made of EPDM. They also say the liquid EDPM sticks good to old EPDM and rubber roofs and the only prep is to wash the existing EPDM or rubber surface. Hope this helps. You might want to ask your question on the RV.NET forums if no one here knows for sure.
Edited to add
I just looked up Geocel Proflex and saw it is a Super Flexible Elastomeric Sealant, most RVer's use Dicor EPDM Lap Sealant with Eternabond tape. Did a search on RV.Net and there are a lot of posts with the term proflex, you might want to check that forum out.