Septic tie in

I did exactly what you're thinking of doing. It was the best upgrade I've ever done! I started out by going to the city to ask permission, see about permits, and hopefully see a plot that indicates where the existing line was located. I was surprised in all three questions: where I live, no permission or permit is required for adding an additional cleanout (considered a "minor plumbing repair"), and no plot exists that has sewer line information on private property. They could tell me where it most likely intersects with other lines in the street, but that's about it.

For the install, as a "do it yourself" project, finding the line was the hardest part. It would have cost between $125-$300 to locate the line with a service. I figured I'd try for a while before resorting to that, and did find it between an existing cleanout right next to the house and the probable intersection in the street. I cut the existing 4" PVC line, and used a directional flexible "T", and piped it up from there. The plumbing was a lot easier than I was expecting. Pics are in this set here. Total cost was around $80, I believe.

1" slope in your 25' is more than enough slope ASSUMING you dump the black first. Plenty of grey water coming next to push any stuff still in the pipe. If you do not have enough depth to put a 90 on the new end, then I'd use 4" sch 40 PVC as your hose will not fit in 3" pvc.

I added an additional "clean out" on my sewer system. I used black ABS pipe to match what was already there.

For 3" and 4" drains, the 2012 International Residential Code, which most U.S. jurisdictions use, requires a slope of 1/4 inch per 1 foot of run. In other words, if you have a four foot section of pipe, your slope is 1 inch. You want to keep to the 1/4"/ft slope and not exceed it either. Of course, if you don't have enough slope, you won't get good drainage. If you have too steep a slope, the liquids will run too quickly, and the solids can be left behind.

Make sure "stuff" will be able to flow down to the tie in.
May have to dig down to the existing pipe, then shot some elevation/grades.
If you have trees around, I would use schedule 40 pipe. Probably, use schedule 40 anyway. I cannot tell you how much slope you should have. Hopefully, someone will know and post.
Pat

I wouldn't call it "creative", but if you can maintain the proper pitch (check local codes) towards the existing pipe, I suggest digging a ditch and burying a PVC 4" hard line. Cut it into the existing line just past the existing cleanout with a directional sanitary 'T' fitting. At the "working" end, just use a 90° fitting with a stub up to a standard 4" screw cap cleanout at ground level. I dug a hole around mine and filled it with crushed stone, so any minor spillage when unhooking the slinky can drain down instead of puddling.

If I had the slope (min 1%) and an easy digging area, I would trench it and install appropriate fittings and pipe runs and have a new service location (cleanout) right where I want it.

Did two at my place.....I used ABS black pipe....the PVC tends to distort and leak after a while, especially if you tend to drive over that area a lot. Just make sure you get a nice grade on it and then that stuff will only run downhill.

if you do it, call it an additional clean out. not a RV dump station.
bumpy

I haven't done it but I would use PVC for a permanent installation.