There is no link from the Travel Itinerary to one or more expense reports.
As for expense to Travel, I do not see a field for getting an itinerary associated with an expense entry. Some clients do require the itinerary to be associated with an Airfare expense (for example), but I don't see a dedicated field for that link. I do see an ereceipt field in the expense v4 but I am unsure what it returns (you can only test in production).
My recommendation would be to associate expenses based on dates as the itinerary does have an airfare start and end date (which might not be accurate with actual start/end due to delayed flights). or use hotel start/end.
