Sorry I haven't gotten back to thank everyone for their input and suggestions. I've been moving so have been purging stuff and packing, purging more stuff and repacking, and so on to try to get everything I need / want for living on the road to fit comfortably in my teardrop and car. I am down the the nitty gritty crap that drives me crazy!

Sharon, thank you for your suggestions and thoughts on what could have caused the issue. My bed is a raised platform so that does help some. The Reflectix pieces I had were just under part of my body, and that's the only place I had mildew issues. I'm guessing it held the moisture there instead of letting it air out normally. I
JuneBug, thanks for the suggestion of the ensolite. I had a camping pad of that years ago - hadn't thought about that as a possibility - would also add some extra comfort to my foam mattress. Though I'm actually thinking of putting some on my floor under the carpet for insulation and cushioning (I have an open dropped floor in my teardrop. That might be really nice on cold mornings!)
Cosmo, thanks for the link to the hypervent - that is so cool! Someone over at TearJerkers also mentioned that. I think that would be a wise thing to have since I will be living in my teardrop in changing climates and environments. I think I will look into that when I can afford it.
Stacie, I almost went with what you did (great minds think alike!) - then I saw the "bag" protectors and thought that might be easier. I thought so until I went to make my bed up. I think it took me at least 15 minutes fighting with the foam mattress and mattress topper and all the coverings. If I went with the bag type protector I'd have to take my mattress out of the teardrop to get it on - which would then require me getting it back IN the teardrop! I think I'd have to have an unsuspecting helper help me with that. Not sure what would be the biggest challenge - getting the mattress OUT of the teardrop or back IN or getting that sticky foam to slide down into a "bag". Probably not a good idea to ask a friend to help me with it if I ever want them to speak to me again! I almost stopped speaking to myself after trying to make the bed!
For now, I did a temporary and FREE fix. First I scraped the mildew off the foam with a wire brush and sprayed it with vinegar (I don't have bleach Sharon) & let it dry. Then I rolled up my Reflectix window covers and moved them to the car - hopefully just that will solve any future problems. Then I tackled the bed. When Egon made my teardrop, he included a 1- 2 inch mattress topper that had a thick mattress pad over it for the bed. I added the 4" foam piece. I used the thick mattress pad he gave me to cover both pieces of foam then a fitted sheet and so on. When I made the bed this time, I added a waterproof mattress pad that my dad and step-mom didn't take when they moved and put that over the 4" piece of foam, then added the mattress topper and it's thick mattress pad over all of it then my fitted sheet. Let me tell you, THAT little experiment was quite a challenge to do in the teardrop without taking any of it out! Needless to say, fighting with "sticky" foam is NOT my favorite sport.
So, I will see how that works for me. When I can afford to upgrade to a mattress protector "bag" or another waterproof mattress pad to cover the bottom of the foam I will do that and I would like to get some of that hypervent too. I think that would be a wise purchase.
Thanks everyone for your input and suggestions. It really helps to hear what others have done and to be introduced to new products out there that can help.
Happy camping!
Jordan