One method that will will work for sure is Liquid Drano. Our owners manual says to use it if needed and have a couple of times. You'll need to tow it around for a bit to make it all slosh around enough. Depending on how bad it is, maybe a 2nd go at it might be needed. Instead of getting someone to pump it out, is there no dump station nearby? You can find them on sanidumps.com.
Used to have a Catalina TT and the black tank was easy to dump and flush and never had trouble with it. Some blank tanks are not so easy due to shape and outlet pipe location.