Here is an online Toned Pinyin input service (free) which allow 1-step input:
(1) go to www.typingChinese.com/main.htm.
(2) Click on the Chn button under and to the left of the big editbox, so that it is in Western input mode.
(3) Hold down a vowel key (a, o, e, i, u, or v), then tap a number key (similar to using Shift key to enter letter). e.g., holding u, tap 2, produces ú; i.e., u + 2 --> ú.
(4) Likewise, e + 4 --> è; v + 4 --> ǜ. o + 3 --> ǒ. v + 5 --> ü.
*Note: 7,8,9,0 (corresponding to tone 1-4) can also be used for both-hand-typing convenience; e.g., to type ā, press a, then tap 7 on the right side of keyboard.
Documentation:
http://www.typingchinese.com/TypingTonedPinyin.htm