Exactly what I was thinking, if not a full set of gages, how about just a green "Full" light? It would make things easier for about $10. worth of parts.
Your Trail Runner most likely has the same setup as mine . . .
At the fresh tank filler, there is an overflow opening next to the water inlet.
When your tank is getting close to full, you'll here it make a gurgle sound . . . and when it is full, it will overflow!
Some people choose to stop when they hear the gurgle sound . . . others fill until the water starts coming out of the overflow.
That is what the overflow is there for.
That meter above is cool . . . but really is not necessary.
As for the water heater . . . I generally prime the water heater and the water lines at home before I leave on a trip so that I don't have to waste water in the fresh water tank to do that.
However, I always try to travel with only enough water in the fresh tank to use the facilities while on the road.
Why carry all of that extra weight if you don't have to?
I have two 100-foot hoses (and a water thief for those state park camp grounds that have those water pumps without the hose threads) . . . plus I carry two water jugs with wheels that hold 13 gallons for when I need to fill up the tanks from the pumps when I can't hook up the hoses.