Ditto on what Mainah says. The rust would have to be very severe indeed before it would compromise the integrity of the pipe wall.
It's an easy job to eliminate the concern altogether. With the tanks valved off (or, better, removed entirely), just go over the piping with a wire wheel or wire brush until the rust is removed, and then cover them with a good corrosion-inhibiting paint like Rustoleum.
This is one of my jobs on our camper for the off-season. The rust is unsightly, and I don't want it to progress any further.
Matthew and Lisa
TT: '02 T-2363;
TV: 2012 Ford F-150, 4x4/Off-road, 5.0L V8, "Big Gray"
Reese Straitline w/ dual cam