A composting toilet might be appropriate in this situation such as
This (Nature's Head) or
This (Air Head) which both require 12V for air circulation and venting. Alternately, there are composting toilets which use no power and basically consist of a seat with a bucket lined with a plastic bag. After each use, peat moss is placed on top of the waste. I haven't used any of these, but supposedly they don't smell and once full, the bag can be discarded, and a new one placed in the bucket. No water, no freezing. With the first two, you would need to prevent the urine from freezing, perhaps by emptying the jug when leaving each morning.