If you want to set PSI the correct way you should have the coach weighed, if not four corners then at least the 2 axle weights, check your tire manufacturer's recommended psi for the weights.
MH tires should be covered when not in use to prevent uv damage. Some take the weight off in winter, I don't. The more you use a tire the more the oils and chemicals keep them in good shape, lack of use and UV damage are usually the culprits to tires going bad. 
Levelling has 2 purposes; one is comfort. The other reason may depend on your slide outs and what your manufacturer recommends. Some require you to be level before you deploy the slides, some say to level afterwards, check with them. As far as damage to appliances not being level, that's really not an issue anymore.