I'll assume the existing drain pipe is just pointing down and is 14 - 20 inches off the ground.
If you want a portable tank, i would select one which was several inches lower than the existing pipe. Then use a short temp extension pipe to reach the tank, could be solid or flexible.
If you wanted to go with a sewer/campground line. Just use some flexible hose at the length you need. If you wanted to go with a standard RV sewer hose, just neck up your pipe to 3.5 inch OD PVC. Then add an RV sewer coupler. If your clearance to the ground gets too close, put on a 90 elbow on the existing pipe first. You can get all necessary plumbing at any hardware, minus the RV specific stuff.
John & Clare - Buck & Zeus - Jack & Husky
2008 Fleetwood Bounder 38P
Towd: 2 HD Streetglides