You say the clean out is about a foot higher when parked in the driveway. Are you talking just a foot higher by driveway, or a foot higher thanthe dump outlet on the RV. My thought is that if the outlet is lower than the dump outlet you could still use your stinky slinky. Gravity will push the stuff up hill until your tank is empty but leave the hose partially full. Then you can just lift up the hose (leave it connected to the RV) and walk the rest of the hose empty.
Another option might be to see if possibly the sewer pipe runs under a planter or the lawn before it gets to the street. On my driveway my clean outs are multiple feet above the level of the trailer. While I was considering a macerator I noticed that the cleanouts were angled slightly toward the lawn and not straight down the driveway. After some digging around in the lawn I now have a cleanout way down near the street that I can dump into.
