There are some reasons for this error.
• Your network is too slow.
• Your server or network automatically kills the process while it downloading the file.
• Your server or network reduces the bandwidth when it downloads a large file.
Please SSH to your server as a root and try this command to check your download speed.
If you can download it, please try to install RVsitebuilder again. If you cannot download it, you need to contact your ISP to check the download bandwidth.
If still not work, please try it manual by following this link.
If finally the methods above don't help, please submit support ticket to us.