UPDATED 11th March 2020: Added straight-through Y pattern, optional flow arrows.

Having a european-built caravan in Australia, I found it very difficult to get the 28mm diameter sink and drain pipe plumbing these are fitted with. I tried a few of the local big name australian caravan parts suppliers (who purport to have the "full spectrum" of pipe fittings) and they didn't know what I was talking about.

I found some being sold on UK eBay but at a cost of more than 48 AUD (approx 32 USD / 28 EUR / 24 GBP) for just one single connector posted to Australia was outrageous.

So, in frustration I decided to do my own workalike connectors. Presented here is a set of 28mm dia fittings for straight-through, tee, Y, straight-through Y, elbow, end cap, U-bend, X, multi-pipe and reducer patterns so you can make your own. Or, you can set it to fit any pipe diameter you like. For acute Y angles the connector length may need to be increased to reduce pipe end impingement. The end cap has a string tie loop added by default but you can turn this off.

The photos show the straight-through, 90 elbow, tee and end cap prints, and how I used them when I needed to re-route the sink drain piping.

Customiser variables
The OpenSCAD script is customisable so feel free to make these for practically any size pipe you like.
Also if you need an elbow angle other than 90, 45, 30 20 degrees just modify the script.
The rendered screenshots show the customiser settings used to produce them. The PIPE_ANGULAR_SEPARATION is only used for the Yand straight-Y fittings and is ignored for the rest.

CONNECTOR_TYPE
straight, tee, X, Y, straight-Y, multi, 90 elbow, 45 elbow, 30 elbow, 20 elbow, 120 elbow, U bend, cap, reducer

PIPE_ANGULAR_SEPARATION
60, 75, 90, 100, 110, 120, 125, 130, 135, 140, 150 degrees

TO_SUIT_PIPE_DIAMETER
Set to your plastic pipe's outside diameter (these connectors go over outside of pipe)

STRAIGHT_CONNECTOR_LENGTH
Overall length of the connector (eg. end-end for straight)

PIPE_OUTERMOST_DEPTH
The first-from-end inside length (at pipe diameter)

PIPE_NOMINAL_WALL_THICKNESS
Change to suit how thick (and thus strong) the conenctor wall is to be. Nominally 2mm thick.

PIPE_INTERMEDIATE_STEP_REDUCTION
The tiny diameter difference inside the pipe, to grip it more firmly (optional, set to 0 if not wanted)

PIPE_INTERMEDIATE_DEPTH
The length of the innermost step diameter.

EMBOSS_SIZE_LABEL
Print the fitted pipe diameter on the outer ends of the connectors if desired eg. "28mm Ø"

EMBOSS_ANGLE_LABEL
Print the angular separation (note - only use for Y fittings so far)

EMBOSS_FLOW_DIRECTION_UP
Print a large upwards arrow on the outside in the middle of the connector length for most patterns

EMBOSS_FLOW_DIRECTION_DOWN
Print a large downwards arrow on the outside in the middle of the connector length for most patterns

TEXT_SIZE
Embossed text size. Nominally 5mm high. Highlight with black Sharpie marker on print.

CAP_TIE_LOOP
Add a small loop to the end cap for tying or looping a retainer string onto.

TO_SUIT_REDUCER_PIPE_DIAMETER
The secondary pipe diameter for a reducer fitting.

CROSS_SECTIONAL_DEMO
Only check this to view the internals of the fitting. Don't use it for rendering or model generation. (see Reducer picture)

PRINTING NOTE
Print at 100% infill. Try and print standing vertically, if possible so that scaffolding may be ommitted on the inside.
These are for sink and drain plumbing only, as the filament I used (PETG) is not rated as food grade. If I had my druthers I would use Nylon, PETG, ABS in that order.
I do not recommend using PLA as that may not remain waterproof.

These connectors and the OpenSCAD script to generate them are placed in the Public Domain. As a courtesy, please leave my name as the author of the program. I do not warrant them fit-for-purpose for anything, so use entirely at yor own risk.