I’m sorry you are feeling upset op but I think you are being a tad unreasonable.
Your dad is working ft, your mum has just had cancer, and they have a side hustle of rental properties they tend to at weekends. That is quite full on,
What you are saying objectively is reasonable, that you wish your parents would prioritise, or even want to prioritise, their grandchild a bit more other than seeing them only once a month or so, but something about the way you put this, saying they can retire and move near you so they are available during the week sounds rather entitled sorry.
You address this in a later post saying you only wrote it down as it is an option but why mention it at all if it’s not something you think they perhaps should have done?
i would hazard a guess that there is something else going on here. Maybe they are just not “baby” people and will come in to their own when your child is older? Maybe there is something you do that is putting them off? Maybe they feel the resentment from you every time they visit that it’s not enough? Maybe they don’t like your other half? Maybe they have limited energy and with your mam having had cancer they want to make the most of enjoying friends and the caravan while they are fit enough? Maybe your mum can’t sleep when she comes to your house if your dc is a bad sleeper and she is prioritising her health atm?
I’m sorry, objectively YANBU at all in that it’s not nice to feel like they are fitting you in out of duty, but it feels like something else is happening in this situation too?
What Is really bad is that you can’t discuss this honestly with them and they made you feel stupid and difficult when you asked them about it before their grandchildren was even on the scene? What were you asking them then?
Can you try and have another proper conversation about it but don’t come from a position of blame, just listen to what they are really saying, and state in an up beat positive way that he is at a lovely age now and you would love them to see more of him? And maybe they could prioritise him once every three months and visit four times a year? It would be a start maybe?
I know a friend of mine says she finds it really difficult staying at her adult daughter’s house because the bed makes her hip sore and because her daughter bosses her about but she doesn’t feel able to say this outright. It could be something like this op?
Whatever happens I would advise you to start trying to gather a few local mum friends together on a WhatsApp group who have dc the same age and start baby-sitting for one another so that you occasionally get that free hour or more you desperately need.
Good luck op. I hope you are able to find some sort of resolution over this whether it’s acceptance that your parents aren’t who they hoped you would be, or whether you find a new way you can interact together?