I'm guessing that you might want some more detailed information. No need to visit the dealer. This is a job you can easily do with hand tools.
Since you described the pipe as thin-walled, I'm guessing it's black (ABS). If it were white it would be PVC, rarely used in RVs. It's getting a little hard to find. Amazon has it. The larger Lowe's stores in metropolitan areas have it. The rural stores do not. Your gray water piping is likely to be 1-1/2".
The easiest way to mend this is to use hubless connectors, also called couplings. Some people call them Ferncos because one manufacturer is Fernco.
Here is one.
Here is another. You will need TWO of these.
Cut out the bad section. Be sure you are well past any longitudinal cracks. (ABS tends to split the long way.) You can use any saw you have handy: wood saw, hack saw, jigsaw, etc. It's very easy to cut. Do your best to keep the ends as square as you can, but they don't have to be perfect.
Then cut the new pipe so it just fits between the two ends of the old pipe. Again, it doesn't have to be perfect, but close is best.
Now, take one of the couplings and hold it alongside the old pipe end so half the coupling overlaps the old end and half extends beyond it. Mark the old pipe at the end of the coupling. You can use a short piece of masking tape or white chalk or whatever. When you assemble this, you will want to have about half the coupling on the old pipe and half on the new pipe. This mark tells you when it's right.
Loosen the bands on the couplers a little bit and slide both couplers completely onto the new segment. Put the new segment into place and slide each coupler out to its mark.
Tighten all four bands firmly. It's easiest to use a nut driver or socket wrench, but you can use a screwdriver. If you do use a screwdriver, use the largest one that will fit into the head. (People mistakenly use a screwdriver that's too small--makes the job much more difficult.)
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