Rhino only has one 'official' liner so far, and when that one is washing i just use a couple pieces of cut to fit fleece as a standby. i guess i wash liners and the other fleece about twice a week these days. have more fleece i could cut though, and then i'd be down to once a week fleece washing.
i pre-soak them in a bucket that has unscented store brand detergent in it, with a little touch of Borax. i get the water all nice and sudsy and plunk the liners in, then swirl and agitate them with a big metal kitchen spoon until my arms are ready to fall off, lol. then i let them soak for a few hours before i toss them in the wash with a load of towels on hot. haven't had a leftover pee stain yet!
i have read that fabric softener causes lesser absorbency of the fleece, but have had no issue with the dryer sheets i use - the odd Rhino pee accident on the fleece soaks right in no problem. the humidity in my place is quite low, so static is an issue, and not using the dryer sheets would mean Rhino gets a shock just about every time he is on his liner!