Septic Holding Tank that's portable?

If you aren't comfortable with running the hose 30' to the cleanout and don't want to pay for a macerator, you can use a portable blue tote and lug it over there 10-20 gallons at a time. It's no fun, but it's also pretty cheap.

From someone who as done all of these suggestion at variuos times and places, Go for the macerator and 3/4" hose, (you can use a 5/8", but get the larger, it is really worth it).
If your conditions change, this will adapt to them.

Tom

If it's only 30' to the septic tank, you can add a 4" clean out in the line from the house by just digging down to the pipe at the closest point to the trailer and installing a 4"sanitary 'Y' fitting. Run a short riser to ground level with a 4" clean out cap on top. If the new "dump port" is downhill from the trailer drain, just add enough sewer hose and an elbow to reach it, otherwise use a macerator as already suggested.

Two options I've used
Portable toilet like the 5 gallon job listed above
And a Barker "Blue boy" Waste tote. they come in assorted sizes and some of the larger sizes have a handle that will fit over a common trailer hitch ball (you don't need a big heavy truck to tow it you could use a small compact car. You do need a hitch and ball) I had I think a 10 gallon and (15 has a hitch adapter optional 20 and up standard) and modified the 15 gallon adapter to use with it. but I can carry 10 gallons if I must. (I can carry 15 if I must). it's just easier to tow 'em. I suggest one that is at least as large as the black tank.

I used mine for Gray water in my trailer days and the 5 gallon port-potty for blaCK.

Just 30' to the septic system? Seems like you really over-complicating this. As already said, just get you 30' of sewer hose (or PVC pipe as one person suggested), and connect the trailer's sewer drain directly to your septic tank clean out.

No need to make it any more complicated than that, especially if its a downhill or level run to the septic tank. If its uphill, then the macerator pump and 3/4" hose would be the way to go. Still would be way cheaper and easier than using a huge, heavy tote tank.

You are fortunate, really. I have much more difficult situation at my house. RV is parked about 100' from septic system clean out, and it (RV) is at bottom of the hill, so its uphill getting to the sewer clean out. Nobody lives in the RV while parked at home, but there are times when we get back from camping trips that I need to empty the black tank.

I bought a 42 gallon 4 wheel Barker tote tank for this. Works OK, but lugging that tote tank 100' up the hill when its full, is brutal! Took two of us to do it and it was not fun, haha. Have only done it once, and decided then that there must be a better way. Thinking of rigging up some ropes and pulleys to pull the tank up the hill, since there's plenty of trees around to anchor to. Still thinking on that one, but point is, your situation is much simpler than mine. :)