I'm sorry to hear this, these things are so difficult to manage and awkward conversations to have. My best guess at the source of the smell (excluding anything obvious such as rotting food or smoking) is damp, is their house well maintained, particularly the roof and guttering? I am guessing they don't regularly open windows, dust and hoover etc to clear the air? Is there any mouldy spots you can see, do they have extractor fans in the kitchen and bathroom and remember to use them? How do they manage laundry, do they dry clothes inside as this can cause musty smells too too. Would they accept a handyman to come around and see if any damp or other structural issues can be identified (if they're resistant you could perhaps use some gentle deception/white lies e.g. saying it's important for their building insurance premiums to do regular maintenance or perhaps you conveniently have a 'friend of a friend' that's a handyman/builder in need of some casual work?). Age Concern are a really helpful charity and they offer fully DBS checked handymen, home helps, cleaners etc who are used to working with the elderly and won't embarrass them about the state of their house or anything like that (they will have to pay of course). They might well accept advice/help from a paid person more so than from you?
Re the personal hygiene, sorry to ask you to be graphic, are we talking toileting issues here or is it just they're not washing themselves or their clothes as often as they ought? Do they have any outside help/carers at all at the moment? It can be a really difficult to go from nothing at all to accepting such personal help with washing/wiping etc but UTIs in particular are really dangerous for the elderly and can really exacerbate dementia so it is important this is taken care of. Would they accept a social services assessment to have professional carers visit and help (it could start very gently if they're relatively able with things like taking them on a brief outing, making cups of tea/simple meals, helping them to the bathroom and reminding them to wash, it doesn't have to go from 0 to full on personal care)? Do they have suitable washing facilities or are they trying to clamber in and out of an unsuitable shower above the bath or whatever? Have them got grab rails and shower stools and other aids to make washing safe? Are they using any incontinence products?
As a very simple measure could you ensure they have plenty of supplies of wipes (I know not eco friendly but needs must), clean pants, tena pads if needed etc so at least if there are accidents it's quick and easy to clean up? Personally I have tried to approach these difficult conversations with elderly relatives as being about them being kept well and living independently at home, maintaining their dignity etc for as long as possible rather than it being the start of a slippery slope to being in a home, losing all control etc which is of course what many people most fear. Best to start having these conversations about their wants and preferences now though and do bear in mind, hard as it is, while they still have mental capacity it's up to them to make these decisions, even if they make unwise ones like not accepting help and putting themselves at risk. The flipside of that is though that they own their own choices and don't put the burden/blame onto you, they don't get to insist that you do more than you are able/comfortable to do, e.g. many stubborn old folks present it as a choice between family become their carers or they're abandoned with no care at all, rejecting the in-between options of buying in help or having state provided help etc.
This board is such a helpful resource, do post more and check out other threads as well, many people in your position and I'm sure people will have good advice for you...