It depends. A reverse proxy, if set correctly, will not reveal information about the web server it points to.
However, there are a lot of things a determined attacker can do to gain information about the origin server:
Your A records surely point to the reverse proxy, but are there other records (typically MX or any other subdomain) that directly point to the original server?
In CDN setups, it's not uncommon to have an 'origin' subdomain that points to the actual server. Sure, you can obfuscate the origin by using a non-guessable name but then often DNS misconfigurations like AXFR vulnerabilities can reveal such domain names.
Various configuration errors or settings on the reverse proxy side as well as on the Apache side can leak the web server's IP address. Some web servers enable the mod-info module which essentially exposes the Apache status page to the internet. A quick Google query reveals many servers with such instances. It's remarkable how many of these exhibit the exact same leakage you are worried about. For example, check out this site in the Google query results. The DNS points to the reverse proxy, but the page shows the internal web server IP.
It's not uncommon for the application itself to reveal the internal IP addresses. For example, can one trigger a SQL error or an 'Internal Server Error' on the application and get the error message to reveal the IP address of the server?
If the application has vulnerabilities like remote command execution or remote file inclusion then it is totally possible to know the IP address (and a lot more). For example, for a vulnerable PHP application a simple
<? phpinfo(); ?> command will spit out everything about the server, including its internal IP address. Similar tricks will work for other frameworks/languages.
There are various other tricks/leakages that might happen. The following article illustrates various ways it can leak:
I've also in the past been successful in getting the origin IP address via https://censys.io/