First, locating your vent. See if this link helps:
https://www.montanaowners.com/forums...ad.php?t=78752
You will have to read through and it also gives some good sanitizing tips.
For many years Montana has not had the old fill style. There are two ways to add water or other fluids - the fresh water fill port and the winterize port. Some have used a hose to pump from a 5 gallon Jerry can or other container to fill their fresh water tank when boondocking. Many of us have a simple harbor freight or other water pump and pump from containers or water bladders.
So, you could mix your bleach solution and pump the solution in using your built in pump and the Winterizing system, or buy a pump and use it and the fresh water fill. Hope this helps.
And last, at the top of the page is a search box. You can save yourself reinventing the wheel if you use it first to answer questions.