Are the drains for the sinks slow too-at the same time as the shower? It all are slow, it sounds as if the vent is plugged and the draining water creates enough pressure in the tank to cause slow flow. Naturally this can mean if the shower drain is the only one slow, it has a blockage.
The OEM tank level gauges are notoriously inaccurate after a short period of use. IMO the easiest and most effective thing to do is use a tank rinser wand.
It comes with a short plastic piece to hold the toilet valve open while using the rinse wand.
This allows you to direct a strong water stream all around the tank walls and blow any debris off the sensors.
The wand is available in many stores, I think WalMart sells them, as well as RV supply stores. If your plumbing is not straight from toilet to tank, this Valterra model
Matter of fact, I just made my own because that Valterra wand is too short(I required one 4' long) for my MH plumbing. If you also choose to make your own, I posted how I made mine in the General Discussion forum.
2000 Winnebago Ultimate Freedom USQ40JD,
ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy