Parsing Grape params in an XML request with complex nested body including attributes

I'm trying to achieve this with Grape. I need to send relatively complex XML bodies including element attributes, something like shown in this sample:

<Travelers>
  <Traveler>
    <AnonymousTraveler>
      <PTC Quantity="1">ADT</PTC>
    </AnonymousTraveler>
   </Traveler>
</Travelers>

And I'm trying to define nested params following a popular convention when converting XML to JSON (here just to ruby) which looks like this:

optional :Travelers, type: Array do
  requires :Traveler, type: Hash do
    optional :AnonymousTraveler, type: Hash do
      requires :PTC, type: Hash do
        requires :_Quantity, type: Integer
        requires :__text, type: String
      end
    end
  end
end

But obviously the XML parser is not following this underscores-based convention and throws the error messages:

[Travelers][Traveler][AnonymousTraveler][PTC][_Quantity] is missing
[Travelers][Traveler][AnonymousTraveler][PTC][__text] is missing

Any clue on how to intercept XML body parameters parser and include this convention logic?