Black Tanks Won’t Flush

Using a pressure washer or power washer.***Link Removed***

Could there be another slide valve located somewhere underneath? I had motorhome which had shut-off slide valve which had to be opened before it would come out the
normal outlet... sort of a safety situation. Just a thought.

Add two bags of ice. Drive for awhile, then empty your tanks.

Trying to understand your issue. Is it that you have 2 black tanks and one drains while the other doesn't? What's a "one way shower valve"? How long have you had this Rv and did it work before? If the 2 connections in the wet bay are unmarked, how do you know they are for the black tanks? And if you have 2 blacks into one outlet, then how do you have 2 black tank outlets in the wet bay? Sorry for all the questions but I am really confused about the issue you are having.

According to the Google you only have one black and one gray tank.

Specs for 2018 Heartland - Bighorn
Floorplan: BH 3870 FB
(Fifth Wheel)

Holding Tanks

Number Of Fresh Water Holding Tanks1
Total Fresh Water Tank Capacity 65.0 gal.
Number Of Gray Water Holding Tanks1
Total Gray Water Tank Capacity 90.0 gal.
Number Of Black Water Holding Tanks1
Total Black Water Tank Capacity 45.0 gal.

I'm betting you have a grey and black discharging together.

Have you properly identified all available connections such as fresh water fill, city hookup and black tank flush ?

Sorry for sounding elementary but not quite wrapping my head around this scenario your experiencing.

Gray tank and black dump to the same outlet
Drain down the one hose,
One 3inch pipe and valve for the black tank,
And a smaller valve and pipe for the grey tank 1&3/4 inch pipe I think !
Personally my RVs have never had a grey tank rinse, I have only had black tank sprayer rinse,
And this RV does not have any at all, I end up flushing lots of clean water down the toilet to rinse the black tank, does not clean the tank sensors, No bathroom window and I don't like dragging a water hose thru the RV to use a spray wand down the toilet,

Two black tanks will have two 3 inch slide gate valves on 3 inch pipes,
Look at the valves , handles, and pipes
One large and one smaller is normal

Perhaps the OP could better explain the problem, because it's about as clear as black water :) I can't tell if its the drain connections or sani-flush or what.

I think he is saying he has 2 black tanks and only one will rinse from the installed sprayer

I think there is confusion , I think he has the , 1 grey. 1 black, grey water tanks don't normally get a rinse sprayer installed in a grey tank

Google isn't always correct due to the info it receives. Our FR 5er lists one grey at 60 gallons. It really is 2 30 gallon tanks with one pull handle behind a round cosmetic port 4 feet from the wet bay and the other in the wet bay beside the black tank pull.
One grey is bath, the other is kitchen.
Looking at the 5er specs you might want to find a scale so that you know what you are dealing with truck spec wise.