As "Walaby" I'm somewhat confused???? First you say the black tank would not drain.
So, you take it to the dealer, he drains the black tank and tells you everything is OK. Now you go back to the RV park and use the facilities.
You then say the sink is gurgling so you try and drain the black tank again but it won’t drain. If the sink is gurgling it’s probably the grey tank that is full not the black tank. Most RVs have the sinks draining into the grey tank.
In addition, if the grey tank is full you would see water coming up into the RV at the low point in the drain system, most likely in the shower basin.
So now you’re ready to go back to the dealer and have him drain the black tank again???
Are you sure how many tanks you have and do you know which valve handle to pull to empty them???? Some RVs have two grey tanks.
I’m not trying to be sarcastic here so please don’t take it that way. What you are explaining just doesn’t add up, sorry????