I do all fresh water in 1/2" PEX, the Sharkbite system of parts are super expensive but very idiot proof. I use the crimp fittings as they are cheaper, but you gotta buy the crimp tool. PEX is the best in my opinion for fresh water, easy to assemble, lasts forever, the pipe itself expands and contracts during temperature fluctuations, doesn't crack if it freezes. They make a 1/2" male threaded piece that fits in most RV water tanks, though some are 3/8" and you will have to buy parts to adapt it over to 1/2". Fill line I use 1" PVC.
Grey water is all 1 1/2" ABS for me. 1 1/2" to 1 1/4" Fernco to adapt it to the stem on the RV tanks. All I use are fresh water style tanks, not the black ABS. I get all my tanks for a scrapper who pulls apart a large number of RV's and travel trailers, thus I get tanks for like 20 a piece. Found it best to use fresh water tanks as they are always clean and come in standard rectangular sizes.
I drill a hole in the tank for the ABS to enter for the drain, before it enters I put a vertical vent. Most codes allow that, a few want it vented to the outside.
Go Youtube PEX crimp fitting, pretty sure you will find a few videos on it. Really easy to figure out, and cheap.
That make any sense?