2006 RBH19 GULF STREAM LITE BLACK GREY PLAYING TRICKS WITH ME

Okay this one's got me complexed.
I bought this travel trailer about 2 and 1/2 years ago I wrapped it and let it sit.
It had pink antifreeze in the lines so I know that it was seasoned because of that, for what I wanted it for I needed to take the back water heater out. I'm probably just going to move it if it works
if I take the water heater out of the equation the inlet source of cold water in and the hot source of output can be connected together as a loop until I put the heater back correct?
When I'm trying to do has Fill the black tank with the blue stuff with some disinfect it down the toilet bowl.
And just trying to bring the blackwater up to about 3/4
I'm trying to do the same with the gray tank except the Grey tank will have a little bit of Lysol disinfectant in it I'm just trying to really wash and sanitize those tanks out I don't know how the previous owner took care of it but I believe they were both empty.
The freshwater I put a little bit of bleach in the tank maybe a cup to about 20 gallons.
At first glance I looked at the furnace and the furnace on the fuse box is missing the blade fuse the one above it is a 40 amp to think that's what the furnace Atwood 5894.  is supposed to have?
In a nutshell
I am getting rid of the tub tub drain tub hot and cold water the little sink in the bathroom by the toilet sink drain is going to be gone, and get rid of the hot and cold lines to the small bathroom sink.
I am now going to keep the toilet obviously the toilet drain and only one cold water hose.
Is there anything special I need to know because I will be trying to reroute or I should say take out two drains that are gray water drains and I will only have the main kitchen sink draining into grey tank.
Thank you for any help in any pointers.

It is ok to connect the cold water IN to the hot water Out. That will give you cold water to the hot water positions of sinks, tubs and showers.

When you remove a drain line to the gray water tank, plug it or cap it. This will prevent odors and water from spilling out if the tank is full.

The 30amp fuse for the heater is probably ok but I would get the model number from the heater and find the manual to be sure. 20 amp would seem more likely. The amp usage will be mostly the fan running.

Good luck

The propane furnace only uses 12v for its fan and that should be 5-7 amps.  The very largest Atwood furnace uses only 9.7A @ 12v, so I would not expect your furnace fuse to be no greater than 15A and 10A is probably plenty.
I can't find any specifics for a model 5984 furnace. Possibly a type in the model number?

Could not find any info on a 5894 either

Your water plan sounds legit.  Just make sure ALL lines are securely capped!

Power centers are frequently mislabeled.  Verify which circuit feeds what and relabel everything.  Pull a fuse and see what does not work.  Replace fuse and pull the next one.  Same for the 120V circuit breakers.

Sorry my bad its an ATWOOD 8531 FURNACE

According to Atwood's specs, the 8531 uses 7.6A @ 12v, so a 10A fuse is adequate but 15A would be OK too.  Do not use 30A or 40A fuses - that defeats the purpose of having a fuse in the first place.