I have a factory installed p trap in a shower/tub. This is in a very cheaply made 2017 mobile home,Think ~30$/ square foot cheap. While obviously there should be water in the p-trap, the problem is, there is visible water in the drain. At the level of the grate, about 1- 1.5 inches below the bottom of the tub. It is not a drain problem, as the water never backs up. Also, it has been this way from the get go. My concern is it will cause the metal to corrode and eventually leak, since I don't think the tub drain is designed to be in contact w water 24-7. My question is; can I adjust the level of the water in the p-trap without redoing the plumbing, or does the p-trap need be redone/lowered?. Access won't be too big of an issue since it is a trailer house. Thanks.

Water shouldn't be up to the drain. You may need to lower your P-trap or you have a high spot in your drain line beyond the trap keeping it from draining down as far as it should.