I've been looking at a full-timing unit for a while as well. In my case, I've only looked into DRV, Excel, Lifestyle and New Horizon mainly because of the advises I received from the forums like this one. Virtually no one argues against them except for the price. One of the most important reason for considering these (despite higher asking price) is because we are expected to move frequently - typical stay will be 2-4 weeks at a time. This will put a lot of stress on the trailer. If we were expected to do 3-6 months stay at a time, these might not be needed. These four units are really built strong with decent frames.
Having said that, I looked at the Landmark on their website. On paper, the spec looks good. The insulation looks pretty good too - even the side walls are R-11 which is better than typical average value of R-7. But I couldn't find any info on their site on how they are built. What kind of frames are they using? Are these guys equivalent (or close to) the 4 I mentioned above?
If you are on a budget, something like Arctic Fox might work out quite well too - they also build their own frames (not as strong, at least in theory compare to the above 4 but still better than most Lippert frames used by almost everyone) and they are 4-season friendly (although I don't get their claims - their insulation is R-7 everywhere!)