If mojoportal is installed as a root level IIS web site it should work with
http://rentals.mainsite.com/SiteMap.ashx
but if you have some kind of funky url forwarding or masking or other weird configurations like GoDaddy offers such that the site is really installed as a sub directory/virtual directory beneath a root level IIS site but they somehow make it seem like a root level site all bets are off and I don't even expect mojoportal to work correctly under those kinds of setups.
See also Submitting Your Site Maps to Google
Hope that helps,
Joe