B+ motorhomes

Gene, at least on mine, it's not the diameter of the tubing, it's the tiny brass right angle stop valve they used. I took it out the other day to see about changing this, because it's an annoyance. I recall the last trip it took forever to drain the tank. I couldn't find any valve at the big box store that would fit in the tight space available. What I do plan to try is to make one like my trailers have had. No valve, just a capped fitting on the bottom end of the tube, outside the RV. Mine would hang down just in front of the genset. But that capped fitting must be an RV item as I could not find it in the big box store either.

Cap
Fitting on end of drain tube

It's not a high priority for me, but I'm going to be on the lookout for the parts when I'm in RV stores.