I don't wear trousers. I'm 5'10" and find that I too have the crotch between my knees, and was constantly pulling my tights up.
Until, that is, I discovered "Silkies". www.silkies.co.uk/ They have all sorts of tights - for fat people, for short people, for tall people. all sorts of types of support [both tummy and varicose veins], and they cost something like £5/6 per pair. They last for ever, unless they are caught on something, the thick ones last for two or three winters. I can't do without them.
They are only sold mail order and you can order no fewer than 4 pairs a month, but you can postpone the order for up to 6 months...
Good luck