I might throw some ideas in to let you think it over?
How familiar with RV water heaters are you? Are you used to hearing them light and heat the water for a bit before shutting the fire off?
Does the fire come back on as you try to use hot water?
What I'm getting at is the idea that the thermostat may not be letting the water get hot enough or if new to the game, possibly not waiting long enough?
Also it may be helpful to know how the hot water changes to cold. Somewhat slowly like the heater is just not keeping up with the use or is it hot and then suddenly cold?
If the hot water is not hot enough and it is only 6-10 gallons, we may use it all up and then it becomes colder as there is a smaller amount mixed with the hot we get at the faucet.
If you turn on a faucet with only hot water, does it start hot and then gradully get cold?
Not getting hot enough, might be due to a bad thermostat. They are often just a small "button" type thermostat on the side of the tank. You might see it sticking out with a couple wires attached?
But there is also a funky, tricky item that can fail in ways to fool us. Do you have a bypass knob like this drawing?
This is often a plastic setup and the shaft inside which makes the actual change in flow can seperate from the knob in weird ways!
It can work well to toally close flow but then it can sometimes not open fully to let the water though to the heater and on to faucets. This can make you start with full hot water in th eline if you've waited but tthen when that water is used to get as mix of large amount of cold with just a bit of hot, making you think it is ALL COLD?
Or is it possible there has been a different bypass setup added over the years? Somebody possibly added an after market type and there are two knobs which need to be set right and you may only be setting one?
Sorry, more questions than answers but maybe we can get the discussion started!!
See if any of that makes sense