The error is caused by the formatting on the strings in the Arrays. They are: <string>something</string>. They need to follow the formatting of the parent: <tem:string>something</tem:string>.
There is a subsequent error that Name is a required field on the Customer table. Also, the SalesRep is a reference to a separate table, be sure to use a real value.
The follow request will work:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:tem="http://tempuri.org/">
<soapenv:Header/>
<soapenv:Body>
<tem:MethodAPIInsertV2>
<tem:strCompanyAccount>{{account}}</tem:strCompanyAccount>
<tem:strLogin>{{login}}</tem:strLogin>
<tem:strPassword>{{password}}</tem:strPassword>
<tem:strTable>Customer</tem:strTable>
<tem:arrInsertFieldsArray>
<tem:string>Name</tem:string>
<tem:string>TenantID</tem:string>
<tem:string>FirstName</tem:string>
<tem:string>LastName</tem:string>
<tem:string>Email</tem:string>
<tem:string>BillAddressAddr1</tem:string>
<tem:string>BillAddressAddr2</tem:string>
<tem:string>BillAddressCity</tem:string>
<tem:string>BillAddressState</tem:string>
<tem:string>BillAddressPostalCode</tem:string>
<tem:string>Phone</tem:string>
<tem:string>AltPhone</tem:string>
<tem:string>AltContact</tem:string>
<tem:string>Branch</tem:string>
<tem:string>SalesRep</tem:string>
<tem:string>VelocifyLeadID</tem:string>
</tem:arrInsertFieldsArray>
<tem:arrInsertValueArray>
<tem:string>TestFName TestLName 2</tem:string>
<tem:string>1</tem:string>
<tem:string>TestFName</tem:string>
<tem:string>TestLName</tem:string>
<tem:string>testemail@test.com</tem:string>
<tem:string>123 Test Street</tem:string>
<tem:string>Apt A</tem:string>
<tem:string>Los Angeles</tem:string>
<tem:string>CA</tem:string>
<tem:string>90005</tem:string>
<tem:string>213-111-2222</tem:string>
<tem:string>310-222-3333</tem:string>
<tem:string>310-555-1111</tem:string>
<tem:string>Austin</tem:string>
<tem:string>RJL</tem:string>
<tem:string>123456</tem:string>
</tem:arrInsertValueArray>
</tem:MethodAPIInsertV2>
</soapenv:Body>
</soapenv:Envelope>
Response:
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<soap:Body>
<MethodAPIInsertV2Response xmlns="http://tempuri.org/">
<MethodAPIInsertV2Result><?xml version="1.0" encoding="windows-1252" ?><MethodAPI response = "SuccessSendToDesktop" RecordID="57306" ></MethodAPI></MethodAPIInsertV2Result>
</MethodAPIInsertV2Response>
</soap:Body>
</soap:Envelope>