When you say had the sewer pumped, it sounds like you have a septic tank system. It sounds like it may have a problem...the leach field may be shot and needs to be replaced. OR, you have a clog somewhere. If a septic tank is not pumped out on a regular cycle, it can fill with solids and eventually get washed into the leach field, plugging it up. How often that is needed, depends on the use and size. A vent system is required for any plumbing system to function properly. It normally goes up and through the roof. Something they could put underneath may not resolve any inadequate venting. A manufacturerd home should have been built with a reasonable venting system, but over time, it could have been cut off and plugged if the roof has been redone and whomever did it, didn't know or care about the ramifications.
If you don't have a septic tank and leach field, and things are backing up, you have a clog in the sewer line, and it needs to be cleared by a competent plumber. The people that did yours may not be, or they didn't go far enough.