Holiday talk in my house is a bit of a moot point as we haven't been able to afford any sort of holiday for several years.
However, when I daydream about it I do fantasise about having a big "family" holiday which included all the children (though 2 are adults) - both DP's and mine - and, if we could afford to do so, and if we had issued the invite (as opposed to them asking if they could tag along) then I'd happily pay for the adult kids. I would, however, expect them to bring their own "spending money" - above and beyond the "usual" aspects of a holiday like excursions, drinks, ice creams (so long as they weren't asking every 10 mins) and meals. Whereas we'd give the younger kids money so they could blow it on tat.
Having said that, if one of those adults was a lazy bum who was making no effort to find work - or otherwise occupy themselves usefully while they looked - either by pulling their weight at home in practical ways or by doing volunteer work, I wouldn't feel inclined to pay .... for their own sake really, as being given stuff on a plate regardless of their attitude and effort wouldn't incentivise them to find work, or make themselves useful.
Disclaimer: I know how appallingly difficult it is for many young adults to find work right now. I wouldn't penalise them for making every effort but not getting a break, no way ..... but wouldn't be happy if they made minimum effort and never helped in the house etc. If OP feels her stepson falls into this category I can understand why she's written this thread. It's particularly galling if - through choice (rather than bad luck) he doesn't have much money yet expects spends when the younger kids who actually are children, and therefore incapable of finding their own money are expected to bring spending money of their own - presumably saved up pocket money and/or birthday/xmas money. If the OP's DH expects his younger kids to do that then there shouldn't be a different rule for the older one. First off - I think OP you need to agree with your DH that the same rules apply to ALL re: drinks and icecreams .... decide how many you think is reasonable ..... and of course, the older "child" shouldn't be favoured above the younger ones.
I do understand that your DH would want all his kids there - but at the same time he should appreciate your resentment if the 20 year old expects everything on a plate but is making no effort to become self sufficient. Maybe that is a wider issue and something your DH should be addressing - as much as one adult can influence another - by having some serious pep talks - and offers of support, advice etc. Otherwise, this sort of issue will just rumble on ......