I have done EPDM on a dozen or so campers or consesion trailers, I even did a TPO on my personal camper. I used the TPO on mine because it was A.) white and reflected the sun. B.) White and matched the camper.
EPDM is fine, I wouldnt worry about wind much at even 75 MPH on a camper roof time membranes hold up to much higher winds. Must be a fully adhered. I have done them both with and with out a 1/2 coverboard. If going right to the metal make sure to clean the crap out of it as alot of htem have various types of seam sealer/coatings and if an older unit the paint chaulk. I replaced all of the vents and skylights because it was easier to buy them then scrape years (mines a 1969 forester) of various coatings off. I think I spend 30 or so for each skylight and probably another 20 or so in other vents. Best part is you can do it with out any seams to leak, even on the biggest 5th wheel.
Break down of what I did.
1.) Power wash the whole top with a mild soap, then spray clean. Allow to completely dry! 24 hours +
2.) Remove all vents/skylights/sat dishes.
3.) Remove all drip rails/awnings.
4.) Lay material in place. (this you might need a hand with unless you have done alot of single ply roofing)
5.) Glue membrane down to substrate, metal or coverboard.
6.) Trim membrane to width and install drip rails. I used waterstop between the membrane and camper. (Tip: easy on the water stop doesnt clean up well off the side of your camper. Also These drip rails make great guides on triming your membrane.
7.) Install Termination Bar on front and back. Same detail as the sides with the drip rail.
8.) Calk the top (Roof side) drip rail and termination bar.
9.) Cut out membrane where vents belong.
10.) Install new or cleaned up used vents down using flat head screws set in waterstop.
11.) Strip in with coverstrip.
On my camper 7'x22' it took me about 8 hour total to do this, not including the 24hour drying time. I did it all after work so in 3-4 hour spurts. It will probably take others longer but this is a good project that can be done insteps and keep your camper dry on the inside.
A note about metal roofs on campers. You can seam metal roofs as alot of the older campers did from the factory, as you can tell they didnt hold up well. Most were done with 4' wide sheets so 5 seams in a 20' camper plus the two on the ends. This would be all fine and dandy but they used like a 1" seam and I belive they relied on an adheasive for the waterproofing. This doesnt last that long because most of the campers life is sits level. One way to do this right would be to solder the seams, except this would only allow you to use Galvanized, copper and a handfull of other expensive metals. While a copper roof on a camper would be cool. I just dont see many forking over the dough for that.
I honestly would sugest putting a membrane roof on it. Infact I belive most newer campers, (dont know for sure will let you know in 10 years when I can buy a new one) come standard with a white membrane roof. I dont know if its EPDM or TPO.
Ok I guess you would like an answer to your questions
Is the EPDM roofing on a camper and on a building the same? Not for sure but I will bet they are.
If they are different which is thicker and will building EPDM work on a camper and hold up OK? I went to a RV forum but couldn't find an answer. EPDM will come in 45 mil, 60 mil, 75 mil and 90 mil. I would guess if they used EPDM it would be the 45 mil (cheaper) Everyone I have done has been 60 mill. Including my own with TPO.
I really want to replace the camper roof with metal but am having trouble finding sheet metal in the 8X12 foot size I need.
Finding that size of sheet would be hard and expensive. Does your camper have EPDM now, and you would rather have the metal?