Welshmama: thrush can definitely affect a baby's willingness to feed, since oral thrush can be quite uncomfortable.
Tashanwrn: I suggest making a thread about your situation so that it gets some individual attention. You're in a tough spot but this sort of thing is generally fixable without you needing to discontinue breastfeeding. RSV could explain the slow gain- sick babies sometimes just don't nurse enough and sometimes they burn through their calories too fast. The whole "we're going to measure the caloric content of your milk" is a load of bullspit. The calorie content of an individual's milk varies widely based on when she expresses the milk, how empty the breast is, how familiar the mom in question is with pumping, the time of day, etc. When a breastfed baby has trouble gaining weight, what matters is not the calorie content of the milk but of what volume of milk the baby is taking in.