Oooff sorry. I assumed you had the tank removed. I haven’t done it on my venture, haven’t had the need, but on my sport bike I ended up siphoning out all the gas I could, scooping out all the crud with paper towels and if memory serves correct I flushed it with water and paper towels or a sponge then flushed with water and shop vac it out once I had a good feeling the vapors weren’t going to be a problem. I don’t remember removing that tank but I could have. Regardless I did have all fuel and return lines disconnected as to not induce water into the system. Dried with towels and let it air dry before putting it back together.
Just picturing in my mind the venture tank and small filler neck. I think removing it and doing a rinse would be best, to drain it maybe put a pan under the petcock area and loosen the screws and let it go. My petcock gasket failed this spring and that’s exactly what I ended up doing after I siphoned out as much as I could. It was messy but it is what it is sometimes.