Following the very simple instructions at https://www.manageiq.org/docs/reference/latest/doc-Managing_Providers/miq/index I proceeded to add the new provider.

It’s all pretty straight forward. Putting in the URL to my Tower server I get greeted with a bunch of errors.

Credential validation was not successful:  {:headers=>{"server"=>"nginx", "date"=>"Mon, 13 Jan 2020  23:42:12 GMT", "content-type"=>"text/html; charset=utf-8",  "content-length"=>"3873", "connection"=>"close",  "vary"=>"Cookie, Accept-Language, Origin",  "content-language"=>"en", "x-api-total-time"=>"0.046s"
.
.
.
blah blah
.
.
status=>404

By default ManageIQ and Cloudforms search for /api/v1 on the Tower server. /api/v1 was deprecated and removed as of Ansible Tower 3.6, see https://docs.ansible.com/ansible-tower/latest/html/towerapi/conventions.html

So, what is a person to do? Hit the google. Eventually I came across this bugzilla item https://bugzilla.redhat.com/show_bug.cgi?id=1740860 and it gave a hint as to just specifying the /api/v2 in the URL I gave to ManageIQ rather than just the base hostname. eg. https://blah…./api/v2

Tried it, it worked! My credential validated and a provider refresh was automatically initiated and all my Ansible Tower templates and inventories were discovered correctly.