My husband and I live in the same time as my parents and his mother. We own our own home and also a rental property.
My parents are 70, retired and fit. They seldom need help with anything unless it's something that Dad needs a hand lifting. (although it's only a matter of time before their property becomes too much for them)
MIL works full time and is 64. We started out helping her do stuff like paint her lounge and hauling away garden rubbish.
Me and DH had a big talk about how it is not our responsibility to maintain anyone elses property for them. If they can't physically or financially maintain their home then they need to move. I also felt like MIL would make "requests" for our help that were really demands that we drop everything and spend a weekend doing her chores on her timeline.
I pointed out that as both sets of parents age and as we have children the jobs will increase and our time will become more precious.
We have an agreement now that we will only do things that suit our schedule. And one off jobs like helping put shelves up, or moving furniture around. We do not want to set ourselves up as the janitors of anyone elses home on an ongoing basis.
It's nice that your DH wants to care for his parents but where do you fit into this?
Why are you obligated to help him look after the property? If he's so keen to do it, why can't he go on his own?
Good luck.