Way out on a limb here but have you looked at the plumbing and parts diagrams to spot where this is located and look for alternates for getting to it?
On this age group we don't get the nice new interactive drawings but simple line stuff. But I am amazed at the quality of the drawings for the accurate pictures they give us.
So if you have not looked, perhaps a look might give some ideas?
I have no idea of the exact setup but it appears the flushing setup may have a cutout in the floor nearby where the pipes go up to the valve? I often use the plumbing as well as the parts drawings as they each give the same info but in different forms which can help if I add the two together!
Plumbing here:
https://www.winnebago.com/Files/File...p36g_plumb.pdf
Scroll to supply lines and then sheet 1 of 5 and look for detail D to check if this might be an option? Also a different detail view on sheet 4 may add some bits of info?
Click this snip for better view or go direct for the big story?
Under floor with holding tank not shown?
This may be easier for you to spot when looking at the RV but it looks like this second snip is turned 180 degrees? First has outer wall to right but second has it to the left side of picture? Likely you can't see it from underneath but I have found several RV that have a loose board section under cabinets which doesn't show up at all unless I really look hard for it.
Wishing for luck says there might be a cutout as drawn here?