After much research, Nick started to seal the caravan roof earlier this year.
He thoroughly prepped one roof edge before applying liquid rubber.
Nick wasn't entirely satisfied with the end result, so the project stalled - and then stopped when we received a Notice the Vacate our then home. All efforts were directed to culling, packing and preparing to move. It was just as well we had the "garden tablecover" because the roof still leaked, though now the issue was more toward the pillow-end of our bed (the unsealed roof edge).
I threw the flimsy plastic cover out earlier this week. It had split in numerous places and wasn't much use. We've had some rain the past few days. It doesn't always cause issues but did the other morning, so I bought a cheap tarp today. It just covers the bed. I'm not sure we'll always sleep under it but we'll definitely keep it handy for wet weather protection.