After several attempts, some params are not correct in soap body. Please check details below:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <SOAP-ENV:Header>
        <m:SessionHeader xmlns:m="urn:tooling.soap.sforce.com">
            <m:sessionId></m:sessionId>
        </m:SessionHeader>
        <m:MetadataWarningsHeader xmlns:m="urn:tooling.soap.sforce.com">
            <m:ignoreSaveWarnings>true</m:ignoreSaveWarnings>
        </m:MetadataWarningsHeader>
        <m:AllOrNoneHeader xmlns:m="urn:tooling.soap.sforce.com">
            <m:allOrNone>true</m:allOrNone>
        </m:AllOrNoneHeader>
    </SOAP-ENV:Header>
    <SOAP-ENV:Body>
        <m:create xmlns:m="urn:tooling.soap.sforce.com">
            <m:sObjects xsi:type="CustomField">
                <m:fullName>Campsite__c.test11__c</m:fullName>
                <m:Metadata>
                    <m:label>test11</m:label>
                    <m:required>false</m:required>
                    <m:unique>false</m:unique>
                    <m:externalId>false</m:externalId>
                    <m:trackHistory>false</m:trackHistory>
                    <m:type>Text</m:type>    
                    <m:length>200</m:length>
                </m:Metadata>
            </m:sObjects>
        </m:create>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>