Atwood Water Heater restricted flow

The access panel were gas valve/burner and PRV is ALL you will see or be able to get too from there in that outside compartment.

The slowly lowering of hot water flow is due to the HOT water outlet check valve being plugged.
The HOT (and cold) water line is on the backside (inside rv) of the water heater tank.

You need to locate the access from inside rv.
Disconnect hot outlet line
Remove nipple on tank (check valve inside)
Replace with new check valve

If you are looking in the access panel and are seeing the relief valve, you should also be able to find the plastic drain plug and remove it.(make sure the heater is off and the water is cool and relieve the pressure with a faucet or the relief valve). After the water drains out, then hook your water hose to the water inlet you normally hook up to on the rv and turn it on to see if you have good waterflow coming in the heater and back out of the drain hole. This should help flush out some sediment in the tank and will make sure you dont have any restrictions for water coming in to the heater. Then turn water off.

The next step would be to blow air into the drain hole while moving it all around. This will blow out additional water and sediment left in the bottom of the HWheater. Replace the drain plug.

There should be an access panel from inside the rv to winterize it. You should not have to remove the refrigerator. Just make a mental note of where the water heater is from the outside and then look for a service panel either inside a cabinet or under the fridge, or like in my case I have to unscrew a panel under the entertainment center, which is likely the case under your fridge, unless it's in the basement area. It may be difficult to get to, but you should have a bypass valve between the hot and cold water lines. turn each of the three valves to place in bypass mode, then turn your water back on with the plug in this time. ( If you have a bypass mode valve in your UDC then you wont have to go to the back of the hot water heater to put it in bypass mode)just do it as usual. Go to your faucets and see if you have good water pressure from the hot and cold side now. In bypass mode it will send cold water through the hot side. If you have good pressure this tells you that you have a bad check valve on the hot water heater or sediment is clogged on the the hot water exit side.

If it looks like the check valve is the culprit you'll need to shut off the water, switch the bypass valves back to the normal position, drain the heater again and remove the line from the hot water side(it will unscrew like a water hose), unscrew the check valve(it will require a wrench) and look to see if it's corroded. You should then be able to poke something through the outlet to make sure it's clear and purchase a new check valve. Make sure it's brass. I think they carry them at home depot or lowes.

Hopefully some others will chime in. I'd hate to put you through all this if I'm missing something here.

Good luck