Waste water

Can anyone help we are newbies and trying to get our first motorhome ready for the weekend, we have discovered the Kitchen sink is taking forever to drain, what should we use to clean out the system is it ok to use a chemical drain unblocker

Couple of thing to check . It might be just that the sink has not been used for a while allowing sediment to dry out, put a good level of boiling water with washing up liquid in it and maybe try an old fashioned plunger . The previous owner may have put tea or coffee grounds down the sink.
Is there a kink in the pipework. Is there a vent from the tank and is it clear. if you are getting air bubbling up trough the drain, it may be an indicator of blocked vent. Open the wast tank drain valve and then fill the sink and see if drains quicker , if its quicker it may be the vent.
I have cleared my own by opening the drain valve, putting small amounts of hot water in the sing and then Blowing it trough it an air line , put a cloth around the end of the airline to block the drain hole to prevent blow back.
If all else fails use the chemical drain cleaner.

let us know how you get on.

Most sink waste pipe run under the floor of the van into the waste tank. Sometimes the clip holding the pipes can break and the pipe sags down and restricts the flow. Have a look underneath, the pipe will be slightly bigger than a mans thumb.
I have used about a gallon of hot water with about 3/4 of a pint of standard house hold bleach. Pour into the sink and allow to drain into the waste tank. Leave overnight and then empty the waste tank into a bucket and dispose into you house sewage system. This is good for eliminating odours from your tank.

B & M bargains.

Just noticed my original post said but and m. Sorry

I was trying to keep that quiet. LOL

That spitting back is from air in the tank blowing out some residual water in the hose connected to the vent at the filler neck , next time continue filling without going to the locker and see if it stops after a short time , when the tank is full it will start to overflow from the same vent.
That is how my system works .