Hi all -
I don't have a TM yet, and the rig I do have is an old (1987) motorhome, so maybe things have gotten better since then!
In my current rig, the gauges are HORRIBLE! I have reset them quite a few times, but they just keep on being inaccurate, in spite of using all the right "gauge cleaning" additives. What I've learned after 3 years is to estimate how much water we've used, & dump accordingly. 2 "Navy" showers & a couple loads of dishes will fill my grey water (tiny little tank) so I dump then. Sometimes I don't estimate so well, and the grey water starts to back up into the tub. No big deal, unless you're in the shower at the time!
Doug, you might want to take the rig back to the dealer and ask them to reset the tank gauge. It's pretty easy - I just fill the tank until it just barely backs up, then twist the adjustment screw a little beyond where it began to show "full." (in my rig, you have to remove the panel cover to do this - the adjustment screws are inside).
Congratulations on your new rig & all the fun you've had so far!