While you can easily melt a foil pan over a hot campfire, yours will be filled with juicy peaches - so assuming you don't over-cook it, you should be safe. 40 minutes directly on the coals might be pushing it though - consider using a gridiron to control the heat by elevating the pan slightly, and stick around to keep an eye on things... Remember, dutch ovens are thick, heavy, and slow to transmit heat, and the recipes reflect this.
I would avoid doubling either the pan or the foil: you'll end up with an air gap between the two layers, which will likely just end up ruining the outer pan. Pay a little extra for the thicker "heavy-duty" foil and you should be fine.
Wet sand between two pans is an interesting idea... If you try it, report back!
My biggest worry here would be burning the peaches before they have a chance to release their juices (no slow warm-up with aluminum - you'll essentially be frying them). If you can elevate the pan, you should be fine; otherwise, consider macerating them first (if you have very ripe, juicy peaches, count your blessings and ignore this suggestion).
Finally, consider that Harbor Freight sells cast-iron dutch ovens for $25. The quality isn't stellar, and it's extra weight to lug around, but you'll probably end up with better food all the same.
Best Answer
The reason you put aluminum foil on something you bake is to block some of the heat. More specifically, you want to block the heat transfer by radiation. This is, you only want some kind of infrared-opaque shield between the heating elements and the food. Of course, it has to be also non-flammable at oven temperatures and either washable or environmentally friendly and cheap, making it one-use.
The best solution is to bake like generations of people did before there was freshly milled aluminum foil available in the supermarket: Get a clay bräter, or a tagine, or a dutch oven. Anything with a lid that is big enough to fit in your oven. Bake your food in it. It is slower, but gives you a much better taste due to slower, more even heating.
The second best solution would be to get some other stuff intended for baking, and use it as a cover. The problem is that baking paper as per Rikon's suggestion is probably not opaque enough in the infrared range - at least it is translucent in the visible spectrum. Maybe two layers of it will do, but it will get hard to work with, because it doesn't hold shape as well as aluminum foil. You could try it if you think it will help. I think that a silicone mat is more promising. It is preferable to use a light colored one of the light glossy platinum silicone - I have a black matte one which is great for rolling dough, but it will probably absorb and re-radiate too much heat.
Both solutions can't beat aluminum in one application: targeted charring protection. For a cake baked in an oven which overheats one corner there is still a chance - if your mat can be cut, you can cover half the cake with the piece (some mats shouldn't be cut because they are internally reinforced with something which shouldn't come into contact with food - if in doubt, contact the manufacturer). But if you are baking a whole bird and the protruding wings are getting overcooked, I don't see a good way to pack them in silicone or baking paper and get it to stay on them while the rest of the bird cooks. There, you will probably have to pack them in bread dough and then just break off the dough when the bird is ready.
I have never made meatloaf in a slow cooker - I didn't even know that you can bake in one - so I won't comment on that part.