2 valves beat as 1?

[happypill]

the PO of my new 79' 30'argosy told me that he replaced the two valves the the grey/black water into one 3" bladex valve. this seems true. first i shut off the bladex valve, then when i continually flush the toilet and look down the hole, it doesnt fill up at all...meanwhile the grey water tank gauge shows that ii is getting fuller. so then i release the valve and all comes out. it's not a big issue while i'm hooked up to a sewer line, but when i go travelling, is my black water tank going to drain into the grey water? is that a problem? i seems the valve on the street side of the trailer that indicates main holding tank...well it's just plain old STUCK. is it a big job the replace that. here's why i ask. elsewhereon this forum, folks say in order to getall the eh-eh "solid" waste out of my tank i should leave the black valve closed..then flush when it has more liquid in it. (this makes sense) however in my case....well you know.