Something is blocking the drain. Sending a hose down the toilet only sends the hose into a 50+ gallon tank .. hows the end of the hose going to find the 4Ē drain hole? It isnít a straight pipe like house plumbing. Your just sending hose into the tank until no more will fit .
Unpleasant as it is you need to work from the bottom .. your system needs a enema .. open the drain and with the water on shove the hose from the drain up into the tank .. all the better if itís a pressure hose blasting away the clog.
There are companies that have a fitting that makes this less messy and do it for you. A drain elbow with a hole for the hose to go through.
When you break the clog be prepared .. have someone ready to shut the gate valve as soon as you can pull the hose back out. I had a guy I knew who had a piece of land ( field ) he took the rv out in the middle of so when the clog broke .... well letís say it wasnít in his driveway !