How can I separate glued-up (ABS/PVC) joints in an RV waste-water system?

I have what seems to me to be an impossible situation. I live in an RV which uses two separate holding tanks for the wastewater that comes from the toilet and the sinks/shower. When I went to open the gate valve to empty the completely full toilet holding tank, the linkage which connects the pull handle to the gate valve snapped off inside the gate valve embly, without draining any of the tank!. The gate valve embly is made of PVC and is glued directly to an ABS elbow which is glued to the holding tank. The outflow of the gate valve is connected to a "T" which is connected to the common drain pipe shared with the other holding tank and it's own gate valve and elbow. As all of these linked fittings and valves occur in a very small space, there is 0" of straight pipe between any and all of these pieces. I cannot cut out any of this embly without completely destroying what a replacement embly would be glued to! It looks like in order to replace these valves and fitting that I would also have to replace both holding tanks themselves, at substantial cost and effort. It also looks to me that the ONLY other way out is if I could separate and remove the ABS fittings which attach directly to the holding tanks. Then I could rebuild this idiotic mess. But how on earth could I remove such fittings that are very well and tightly glued on. (Also, there is very little room to work in and it is not possible to access the part of the fittings that are nearly ed up to the underside of the bottom of the RV.) I should add that I was eventually able to drain the toilet holding tank by cutting into the gate valve embly itself with a hot knife and removing a section of plastic which allowed me to grab the edge of the actual gate itself and slide it open. Of course, this means that the gate valve is destroyed and must be replaced. But that was already the case when the opening handle linkage snapped off and started this nightmare. The only solution short of replacing the tanks that I can think of is to use the "hot knife" technique to slice out a piece of the glued up fitting and then see if I can use a hammer and chisel to loosen and pop off the rest of the fitting. But that method sounds fraught with complications and chances of destroying the work. So what the ????? can I do to solve this dilemma?