I've built a craft with engines that require highly pressurized tanks. The tanks (stock tanks, all are by Kerbodyne; repainted with painting mods) have an appropriate setting for it. I have seen other tutorials (A, B) that say about a "tank type slider", but there is only a button which opens a rather useless menu that only provides the "Default" (apparently "unpressurized") option for ALL (mod and stock) non-RCS tanks (I don't want to rebuild all my ship(s) to replace all their tanks with the few available RCS tanks). I'm using KSP 1.12.5, RSS/RO and Real Fuels (all of them are the latest version).
Video: https://files.fm/u/x9sjf5rnqq#/view/u5v48cpju9