We have also had a splash of water when removing the dump cap. We assumed something was broken or cracked. We had a big can to catch the splash for maybe two days and then it was all back to normal. We were miffed but believe that it had something to do with a previous sewer dump spot that was higher than our dump valve. We have come across that problem several times. It also may cause a bad odor inside the Escape temporarily. It is probably just not completely dumping.
Yours is obviously a worse problem. At least you have determined the cause.
Cathy. Floating Cloud
"Live in the sunshine, swim the sea, drink the wild air.... "