RCS+ is a mod that revamps and extends RCS systems and fuels. Stock parts have been modified and new parts added to make RCS more intuitive to use and to create an interesting dynamic between monopropellant engines and LFO engines. So what does it do exactly?
Large RCS thrusters use LFO
Instead of having to put unwieldy monopropellant tanks onto your spacecraft, you can now just put your RCS blocks and thrusters onto ships and stations with LFO tanks and they'll work. This is more intuitive and allows you make much better looking spacecraft. In real-life large spacecraft also often use bipropellant to fuel their RCS.
Small RCS thrusters use monoprop
Monopropellant tanks, with their generally small size or radial attachment, are far more suited to designs for small spacecraft or probes. In real-life, probes often use monoprop thrusters and RCS.
New RCS parts
The original RCS parts weren't sufficient for this mod, so a total of 7 new parts were added to complement the stock parts. Five of them are large LFO-fueled RCS parts with different shapes and nozzles: 2-way, 5-way and 1-way thrusters and 2 variants with offset nozzles. The other two parts are small monoprop RCS blocks: one 4-way and one offset 4-way block.
Reinvisioned stock RCS
The 4-way RV-105 block has been changed into an LFO-fueled block and renamed the RV-104 (for naming consistency with the new parts). The linear RCS port has been shrunk to complement the new small monoprop-fueled RCS blocks.
LV-1 engines are now monoprop engines
They already had similar Isp/thrust compared to monoprop thrusters and they complement the monoprop tanks far better than the couple of small LFO tanks.
Other small tweaks
A few small stat tweaks have been made to existing parts to make them more consistent with the new ones or to fix unwanted behavior. All of the thrusters are no longer massless, which should fix issues with overpowered thrust-to-weight ratios.