Big Country sewer system schematic

If I read right, you have a 90 gal gray which is probably the shower and kitchen sink. The 45 gal black tank is probably your bathroom sink (aids dilution) and toilet. I'd start with pulling the shower drain and look in that trap 1st. Is the kitchen sink draining properly? Blockage where the shower connects into the sink drain line is probably not as common unless there is a sharp bend. I went to Heartland site and did not see a diagram for the plumbing (just flush system and heated pads)but I suspect the different Heartland models are plumbed similar based on floor plan. Since the kitchen sink and shower share a common wall it makes sense to use a common gray tank. Hope that helps.