If you hold this money in USD and spend it in the US as USD, then there is no tax liability or reporting requirement at all. You are not subject to any tax on foreign exchange gains and losses because you have not performed any foreign exchange. CRA says a foreign exchange gain or loss happens when the fx transaction occurs—not as the currency’s value fluctuates while on deposit - and since you are never performing an fx transaction, no such issues will arise. The CRA is not interested in how you spend your money, only the money that you earn.
The only possible tax liability that would arise in the circumstances that you describe would be the tax liability arising from interest earned while the cash in on deposit. If this interest exceeds the threshold of reportability, then your bank will issue you with a T-slip to be included in your tax return.