My father is 85 with dementia. Health is steadily declining. He has a feeding tube, mobility is starting to deteriorate faster. Thoughts?

Not necessarily, no. There is loss of sensation often, and of control, with age. Nothing to do with dementia. Happens to elders who are not demented as well.
The basic problem here is likely to be the fact dad now has to get his nourishment from tube feedings. These tend to "run through". There is little bulk or fiber to form a regular movement. Do check this out with nutrition consult if MD can order this or with MD himself or herself. There are many kinds of tube feedings and some cause more constipation, some more diarrhea. Though they are usually problematic in one wise or another.