Where is the black water tank located on the 1994 tour master by Gulfstream 40feet RV and where is the release handle located?

Question edited for clarity.
Question moved from Cars and Trucks to RVs.

Yes, remodeling is exactly that, pull it all out and start again.

https://www.google.com/search?q=2006+Gulfstream+Cavalier+RV.+Owners+manual

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Question edited for clarity, FULL Maker Model name.
Second post edited in, and Deleted.

Right under the fill door.

https://www.google.com/search?q=2005+pursuit+rv+water+tank+location

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There should be a compartment on the outside, typically where most utilities are connected, that will have the drain connectors for black and grey water. Black is for your toilet, grey is all other water water. Look for the labels to ensure you have the correct one. Depending on the manufacturer, it will be either a black, plastic or silver, metal T-handle. Once located, you will pull the T-handle out, approximately 4-6 inches (10-15 cm).

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The RV sewer system should end in a 3", 31/2" or 4" black plastic fitting that has 2 to 4 plastic tabs that a flexible hose will lock on with a twist. The fitting probably has a cap twisted on. After the sewer drain hose is connected to the RV drain, and secured in the sewer, a 'T' handled pull rod should open the gate valve. Most drain the black tank first, then use the grey water tank to flush the hose clean. If the 'T' handles aren't in your sewer compartment, you'll have to look under the bottom edge of the RV to find them.

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Pull bottom plug out of hot water heater tank let all water drain. Then open all water tank drain valves and be sure your RV is tilted so it will drain towards the valve. Go in RV access water heater valves put it in bypass so no antifreeze can enter water heater tank. Pump RV antifreeze with hose provided into all water lines throughout RV by turning on your main water pump after you have stuck the bypass hose in a new and full antifreeze bottle, then flush toilet until red antifreeze comes out. Open all faucets one at a time until red antifreeze comes out. I use 3 gal but some only use one, the more the better it’s cheap at Walmart. Don’t forget to run outside shower until red antifreeze comes out also, very important or it will crack in the cold. Finally make sure black and grey tanks are empty and flushed at your last camp trip before winterizing. Pour antifreeze in toilet and all drains making sure first that black and grey tank shut off valves are pushed in and closed. Get a quality cover and store RV, it makes a huge difference on protecting roof and roofs lap sealant! All RV antifreeze is non toxic and you just drain it all at your first camp stop after winter! Note: don’t forget to put water heater ionizer drain plug back in wrapped with new white Teflon tape and take water heater valves out of bypass! Draw a diagram of regular mode and winter bypass mode so you don’t forget especially hot and cold water heater blender valve between the two MUST be shut off after you unwinterize
or you will cool your hot water unwanted and have no hot water to use! Happy Camping folks!

U SHOULD HAVE A BLACK TANK AND A GRAY. TOWARDS THE BACK WERE U HOOK UP THE HOSE FOR DUMPING U WILL SEE TWO LEVERS ONE IS FOR BLACK ONE IS FOR GRAY. AND IF UR TALKING ABOUT FRESH HOLDING TANK LOOK UNDER NEATH AND U SHOULD SEE A PLASTIC NOSEL WITH A TURN VALVE LIKE WHAT BOX WINE HAS TO PURE A GLASS

You have the question we are all asking.
rv tank gauges never seem to be very accurate. The best you can do for your black water tank is to try to see where the sensors are mounted and try to spray water on them to loosen any gunk.
I have had 4 motor homes and never had very accurate tank indicators.
Your black tank can be checked by taking a flashlight with the water off and looking down the hole of the toilet. When it gets close to the lowest part,of the drain hole it is time to dump.

You can switch it out but pretty labor intensive. You cannot just add a gas option to the unit. You will have to replace it with a gas heater and then plumb it to the tanks. The existing unit probably has something simple like a safety sensor that went bad or a heating coil.