I have already submitted my application for LTD and am considering selling some RRSP investments to survive on in the meantime. I did not indicate I would be receiving this income on my LTD application as at the time I had not decided to do this. Would this be a problem for my application? I currently have no income

You would need to review your specific LTD policy to see what your policy has listed as sources of income that could impact your benefits. You can also has your adjuster directly if that income would impact you in accordance to your LTD policy.
If you don't have the policy, your insurer has a legal obligation to provide it, so you can ask them directly for that.