I don't know what year your RV is or your financial place in life but if it were me, I would get some contact cement and glue those pieces back down as flat as I could get them. Then go over each area with Dicor self leveling lap sealant to cover the seams. When that's all dry, i would then go over all of it with Eternabond tape and call it good. The alternative is going to be expensive. I just read online that the average cost to replace a roof is $300 per linear foot.
If you have areas where the roof material has completely ripped off, Use the widest Eternabond tape you can buy.