When configured correctly, reverse DNS shouldn't be a problem with multiple domains on one server. Every ISP on the planet sends billions of emails a day from hosts that handle email for hundreds or even thousands of domains.
Let's say you are an ISP called myisp.net, and you have a mail server, mail.myisp.net, on 1.2.3.4.
You can have as many domains as you like hosted on that server, eg domain1.com, domain2.com, etc.
These domains can either have domain specific MX records that resolve to 1.2.3.4, or they can all point MX at mail.myisp.net.
Code:
domain1.com MX mail.domain1.com
mail.domain1.com A 1.2.3.4
OR
domain1.com MX mail.myisp.net
Sending is where rDNS becomes important. Your server has a hostname of mail.myisp.net. The A record for this hostname should resolve to 1.2.3.4, and you should have an rDNS entry for 1.2.3.4 that resolves to mail.myisp.net, so everything ties up neatly.
When sending emails, the destination server will see a connection from 1.2.3.4. During the start of the SMTP conversation your server will identify itself as mail.myisp.net (SMTP HELO/EHLO). The destination server doesn't really care what domain is in the email itself (until you start getting into things like SPF), they only care that mail.myisp.net ties up with the IP the connection came from, and that the rDNS matches:
Connection came from 1.2.3.4, identifying itself as mail.myisp.net
1) Make sure A record for mail.myisp.net resolves to 1.2.3.4
2) Make sure reverse for 1.2.3.4 matches "mail.myisp.net"
The important thing to understand is that everything relates to the real hostname of the server. The domain names used inside emails is fairly irrelevant. You can't start trying to make the SMTP server itself use different names because the reverse DNS will never match up.