There's no legal requirement to have the sticker. Some (a very few) campgrounds might not let you in without the sticker, at least in theory. I suspect in practice nobody would even notice since this presumably is obviously a commercially produced RV.
If you look at the sticker, all it says is that the manufacturer was a member of the RVIA and that they certify that the RV meets the applicable codes (electrical, plumbing, DOT, etc.) in force at the time of manufacture. While it doesn't say so on the sticker, it does also mean that the maker paid a fee to the RVIA per sticker, somewhere in the vicinity of $75 to $80 I believe. Removing the sticker in no way alters the code compliance. It's perfectly legal to build and use an RV that does not have the sticker, and indeed any homebuilt RV will fall into that category, as well as those made by at least a few of the smaller RV makers.
I don't think there's a way to get a replacement sticker, at least not easily. They don't want them generally available for people to put on RV's that don't meet the codes or (probably more to the point) for which the $75 to $80 fee has not been paid. Your best bet might be to work through the RV manufacturer.