In an up-to-date corpus of 20 varieties of English, roadmap is about twice as common as road map. In a corpus built in 2104, the two forms were even-stevens, just about, but by the time of a 2018 corpus the ratio was 3:1 in favour of roadmap. It’s a historical process that has happened repeatedly. When Jane Austen wrote any body she did not mean 'any old cadaver'; body in her sense meant 'person'.
Modern usage follows Shakespeare. In the GloWbE corpus (Global Web-based English), in the meantime is 20 times more frequent than in the meanwhile.
Two poetic "meanwhiles"
And, as I was writing this, the last words of Auden’s Friday’s Child, in memory of the German theologian Dietrich Bonhoeffer, floated into my head:
In summary: champ is older as a verb in its own right, by anything between 51 and 183 years (depending on which source you go by); ‘the dictionaries’ agree that chomp is a by-form of champ; three major English dictionaries define chomp by reference back to champ; chomp in conjunction with bit is actually recorded earlier (1645) than champ at the bit, and the subsequent OED citation for chomp also includes the word bit; and Merriam-Webster shows an intransitive chomp meaning – ‘to be eager (to do)’ – that neither Collins nor the OED does.