Hello Markymarq, and welcome.
Sorry I didn't see this earlier. I've been a fairly similar situation to you over hte past 3+ years; a US citizen living and working in Canada while having no US income. We now have money tied up in both systems.
First, a good resource on how to invest in Canada is the
Canadian Couch Potato series.
Second, you are correct that the US and Canada have a tax treaty. When filing your US income taxes (something you are required to do even while living abroad) just use form 2555 "Foreign Earned Income". The first $100,800 (USD) of earned income in Canada will not be taxed. Ditto for anything in an RRSP. If you are investing int he broader market within Canada (e.g. in eFunds), you'll pay capitol gains taxes there and that also won't be taxed in the US. Finally, you still get your standard deduction of $6,300/year in the US even if you aren't earning any money there.
Third, I think you are falling into the 'currency equivalence trap'. Just a few years ago the currencies were about equal, but since WWII they have varied from about 0.59¢ to $1.1, with our current levels (roughly 78¢) being very close to the mean. However, since these are two large economies there's no 'reversion to the mean' here - we could certainly see the exchange go much lower or much higher, all depending on how Canada does relative to the US. Many people try to make money on currency exchange, and very few do very well. Plus, we are highly influenced by 'recency bias'. All we hear right now is about the 'weak loonie' - but it's weak compared to where it was 3 years ago. Compared to 2001 it's quite strong today. It's all about perspective.
All that is to say that you should transfer money only when you need to transfer it. You can't control or predict the exchange rate in the future, so don't try.
Final thoughts:
i) yes max out your contributions to your RRSP. THat will provide you with a tax shelter and tax-free growth. The worst thing to do is to just let huge sums of cash sit on the sidelines like idle employees goofing off.
ii) convert funds to USD as needed and in the method that makes you most comfortable. Since this isn't a case where "the market always goes up!" DCAing your conversions is a fine strategy and will even out the bumps.
iii) if you have any money in the US in a tIRA start a roth pipeline. You might not be able to contribute but you can use your standard deduction to pay NO taxes on a conversion while living abroad. It's the one silver lining to not being able to contribute.