Back pressure not due to sludge in tank
Since you ONLY feel the back pressure when the pump is in the "dry" position, it means that something has failed in the toilet itself, or you'd feel it in both positions. If the toilet is a Jabsco, it's the dry/flush valve. You're still trying "armchair" trouble-shoot instead of rolling up your sleeves and doing the work necessary to find the cause of your problems. However, something is preventing it from being pumped out...and even if it's overflowing out the vent when you flush the head, that doesn't mean the vent isn't partially blocked. The pumpout line may have a blockage...there may be y-valve in the pumpout line that's in the wrong position...a kink in the line.... But neither I nor anyone else has the psychic power tell you what the problem is without pulling the system apart and finding it. The best I can do from MY armchair is to tell you that I've never seen a tank completely filled with sludge so thick it could not be pumped out--an inch or two on the bottom, but that's all--and list all the possible causes. But you (or someone) is gonna have to pull the system apart to find it.