Bigger's suggestion brings to mind our newbie RV experience when we finally decided to go "wet camping" i.e., load the tank with fresh water and dare to use the commode! We had hauled this old RV all the way from Virginia to Texas and back, then to the mountains of West Virginia during the winter, and we never needed to put water in the lines, as we always were booked in full service campgrounds with decent bath houses....
Well.
So I had to have a check-up colonoscopy last summer, and the hotels up at Cleveland Clinic don't allow patients to bring dogs...we had 2 big dogs. You gotta start drinking the prep and 'visiting the facility' the night before the big procedure, so we finally got to truly test the capabilities of RV camping - just like at home, haha!
I thought it was great to not have to be in a cold, clinical hotel during that looonnnng night with the many required sips of lemon-flavored "prep." (I "slept" on the dinette made into bed, right across from the small refrigerator and just a step back to the bathroom...I didn't even wake up the older dog snoring on the floor!)
Everything was great....until we were on the ride home, stopped at an interstate RV dump station, and discovered that whoever had traded the old fifth wheel unit in must never have flushed out the black tank...and the dealer hadn't done that liitle task either, before they sold it to us! The black tank appeared to have a total blockage of petrified poop & paper. Oh my....
We were told to put a couple dishwasher detergent tablets, a cup or two of septic tank cleaner enzymes, and a whole bag of ice right down the commode, and then we drove the last 100 miles home. Let her sit for a week and then went to a nearby RV campground, paid a $15 dump fee, and everything was fine. Lesson learned....
Gotta, I hope we aren't giving you TMI....it really is unlikely you'd run into this issue with any late-model rental. But maybe worth asking the rental dealer if they have serviced the unit in that fashion!