'What is the difference on the 'bighorn vs the other units like landmark?

'explain to me the difference between the bighorn units vs the other units like augusta, landmark etc.

has anyone every had one of those walk in spa tubs installed in a trl?

The Augusta, which is or used to be, part of the Landmark series, has nicer interior features and is basically a step up from the Bighorn. They use the same frame and suspension and construction materials for the main body. The LM has a full body paint option and gell coat is standard where in the BH it's an option. No paint in the BH either. You might consider going to the Heartland website and look at the photo galleries for both. The differences are fairly obvious.