I have a mobile home single wide older 1978 snaked auger type 25 feet kitchen sink drain still clogged , can I disconnect the pip under home near toilet and then try to force water through it?

Generally, on fifth wheels, the fresh water tank is in the rear area of the rv, but the pump is usually up near the front near the rest of the plumbing. Ie water heater, bathroom, etc. There are storage areas in the front, usually, the pump is on the side where the door for entering the trailer is. Open the storage access door that is on that side in the front and look at the panels that close off tbe storage area to the living quarters of the rv. There is usually a hole for access to a “winterizing point” marked. There us where you will find the pump

Disconnect clear water supply/hose. Turn on 12v pump. Turn on the sink faucets and open the grey water drain. Wait until the faucets start spitting water (you'll know).
Complete.
If you wish to split hairs yes there will be residual water left somewhat in the system but the tank will be emptied. Good luck and happy camping

You'll probably have to crawl under and trace where they go. You could possibly put some dye in one of the grey water tanks (ie: the kitchen sink drain or bathroom sink or shower drain) to figure out where it comes from. The black tank is your toilet drain. It always has the larger diameter pipe going to the valve (usually 2 or 3 inch diameter) The grey water tubing is usually 1 1/2 inch tubing.

This is not just for your camper, but general troubleshooting.
Drain the water from the belly pan. Hook up water supply to shore water source. With water source turned on, allow some time, 1-4 hours, and check if water has returned to belly pan. If water is present then the problem is with the fresh water system. If no water, either it is a small/slow leak or the problem is not with the fresh water system. Next test the grey water tank by pouring some red food color into the shower or sink drain. Flush it into tank with water. Again, wait for some time, maybe over night, and then check for red water in the belly pan. It may take longer for the leak to appear as the grey water tank does not have pressure. If no water, check the black water tank with green food coloring and same technique. If there is colored water, red or green, in the belly pan, then the leak is with associated with the grey or black water tanks. If no water is present after these tests, then the leak may be very small and it may take days or weeks to have water re-appear. In any event, clear water is fresh water system, red water is grey water system, and green water is black water system. With any of them, checking the tanks themselves and all pipes and connectors to/from should find the leak source. The last possibility is that water is entering the belly pan from a roof or vent leak. Check to see if water returns to belly pan after a good rain. If so, the water may be clear or slightly dirty. If it is a roof leak, I have found the best repair is to use Peel and Seal roll roofing It is light weight laminated aluminum and self sticking. Cutting and applying narrow strips 4-6" wide, will seal any caulked area. It costs $95 (3' x 33') at Lowes. A 30" motor home can be completely covered with three (3) rolls, in half a day. I have re-topped 5 motor homes (one 7 years ago) and no more leaks on any of them. The product was developed to seal the roofing joints of mobile homes. Topping should last 15+ years.

guessing here

one would be to the fresh drinking tank
leaves two to choose from
does the RV have two types of taps ?
how many water tanks on this model ?

have see one layout with chemical toilet and normal house taps at its sink
or kitchen sink with multiple taps

Location varies with RVs, but one of the most common is somewhere close to the kitchen sink. Follow the cold water supply back from the kitchen sink and you should find it fairly quickly.

underneath or they are clogged

Is your tank damaged? It is not an easy job. In most cases you have the kitchen sink, bathroom sink and tub/shower draining into the tank. You will also have a vent pipe coming out. All of these will come out of the top of the tank. You will have to find them inside the coach under the tub, inside a wall or under a cabinet. Then you will have to disconnect the monitor wires under the coach at the tank. Then disconnect the gate valve. Then you will have to remove the angle iron or cross member that is holding the tank up. Once all of that is done you will be ready to wrestle the tank out. Keep in mind that there may be some sludge inside of the tank and it will smell pretty bad when it spills out. Good luck!

They are usually mounted behind a cover panel behind the water fill connection in the under belly storage. Turn the pump on and open the water valve on the kitchen sink and listen for it to start vibrating.

On some later model Montana 5th wheels, there are actually three holding tanks which will eventually require draining. (two grey tanks, 1 for the kitchen sink(s) and 1 for the bath tub & bathroom sink, and one black tank, obviously the toilet...) The two most used drain valves (1 black & 1 grey) are together in the docking station area where the city water supply is hooked up, the other (kitchen sink) is at the left rear of the coach behind the farthest rear wheel along the frame. (That is where "Keystone" installed the grey tank for the galley, or kitchen if you prefer.)
If certain it is the black tank which won't dump when the black tank valve is pulled, there is a restriction before the valve (or the valve itself is not opening hence a broken cable to the valve?) One might try "snaking" up into the sewer line, measuring the distance from the outlet to the valve location, assuring it is either open or closed. Once that has been determined, you will at that point have to remove the sewer line at the valve for repair. Be sure to wear latex gloves and eye protection and have a drain pan or bucket handy for spillage which can be collected and dumped down your home toilet or outside sewer clean out if you know its location.