what are the plaintext's for the RC4 stream cipher test vectors in RFC6229? https://tools.ietf.org/html/rfc6229 has stuff like this:

Key length: 40 bits.
 key: 0x0102030405

 DEC    0 HEX    0:  b2 39 63 05  f0 3d c0 27   cc c3 52 4a  0a 11 18 a8
 DEC   16 HEX   10:  69 82 94 4f  18 fc 82 d5   89 c4 03 a4  7a 0d 09 19
 DEC  240 HEX   f0:  28 cb 11 32  c9 6c e2 86   42 1d ca ad  b8 b6 9e ae
 DEC  256 HEX  100:  1c fc f6 2b  03 ed db 64   1d 77 df cf  7f 8d 8c 93
 DEC  496 HEX  1f0:  42 b7 d0 cd  d9 18 a8 a3   3d d5 17 81  c8 1f 40 41
 DEC  512 HEX  200:  64 59 84 44  32 a7 da 92   3c fb 3e b4  98 06 61 f6
 DEC  752 HEX  2f0:  ec 10 32 7b  de 2b ee fd   18 f9 27 76  80 45 7e 22
 DEC  768 HEX  300:  eb 62 63 8d  4f 0b a1 fe   9f ca 20 e0  5b f8 ff 2b
 DEC 1008 HEX  3f0:  45 12 90 48  e6 a0 ed 0b   56 b4 90 33  8f 07 8d a5
 DEC 1024 HEX  400:  30 ab bc c7  c2 0b 01 60   9f 23 ee 2d  5f 6b b7 df
 DEC 1520 HEX  5f0:  32 94 f7 44  d8 f9 79 05   07 e7 0f 62  e5 bb ce ea
 DEC 1536 HEX  600:  d8 72 9d b4  18 82 25 9b   ee 4f 82 53  25 f5 a1 30
 DEC 2032 HEX  7f0:  1e b1 4a 0c  13 b3 bf 47   fa 2a 0b a9  3a d4 5b 8b
 DEC 2048 HEX  800:  cc 58 2f 8b  a9 f2 65 e2   b1 be 91 12  e9 75 d2 d7
 DEC 3056 HEX  bf0:  f2 e3 0f 9b  d1 02 ec bf   75 aa ad e9  bc 35 c4 3c
 DEC 3072 HEX  c00:  ec 0e 11 c4  79 dc 32 9d   c8 da 79 68  fe 96 56 81
 DEC 4080 HEX  ff0:  06 83 26 a2  11 84 16 d2   1f 9d 04 b2  cd 1c a0 50
 DEC 4096 HEX 1000:  ff 25 b5 89  95 99 67 07   e5 1f bd f0  8b 34 d8 75


So it's pretty obvious what the key is but what is the plaintext supposed to be? I assume each the DEC part of each line corresponds to the offset?
RC4 is some kind of pseudo random number generator. You input a key (seed) and now can get a stream of pseudo random numbers. The values here are this numbers, always a byte at a time.

Unlike a block cipher, you don't have a plaintext. You use this key stream to add it to the plaintext to get the ciphertext. "Adding" can be done with XOR, because that's pretty easy and the most common way.

Yes, DEC means the offset. The first 16 values are the 16 output bytes of RC4 with the mentioned key. The next 16 values are the output bytes after the first 16. The third line has the output bytes after the first 240 bytes.

We could say that the plaintext was all zero bits. This would lead to the ciphertext being the key stream.

This online tools allows to encrypt / decrypt something with RC4. Switch the input and the key to hex numbers, write only zeros as input text and then copy the key into the key field (without the 0x at the start). Now you can see exactly the mentioned numbers as output of the function.