Take the tee off the water line and put a valve on instead. Then just turn the valve to the “off” position.
For the black tank, get the proper size “test plug”, stick it in the end of the pipe and screw it down tight.
just looking at your picture again, is there a panel you can pull on that shelving unit, or does the skirting on the tub come off easy? that plastic pipe would be better off being replaced frrom the source but if you can't get at it then manybe just put a plug cover on, or add a valve in its location. usaly there is a way to get at plumbing.
That's what I meant when I said I'd replace the flex tubing with Pex. The whole length from the source, because it's likely this flex tubing goes back to the water pump (or a junction off of it) (and this flex tubing is probably also connected to the water heater, kitchen sink - everywhere) In this case you could probably get to most of it for the toilet supply. Those kick panels under the shower usually come off easily. I'd be as intrusive as I'd need to be because when it develops a leak you'll have to do it anyway. OP never came back so probably a moot point and we have no idea what solution was actually applied.
FWIW - been looking at new RVs, specifically Super Cs and I have been amazed at how many "high end" manufacturers use this flex tubing crimped to improper fittings. Even on the water heater which experiences all kinds of expansion/contraction. Tiffin, Newmar etc. atrocious. Helping me narrow down to Renegade. They seem to be the only Super C manufacturer that not only uses all PEX and fittings, but shark bite fittings on the pump, water heater etc to do repairs without having to cut the lines. As I get older I am more interested in what I can't see on RVs than what I can.
