Ahhhhhhh Forest River water tanks.
Fill it up, drop the under liner, how much is the tank sagging down? Possibly about 30 gallons worth of sag in the middle? Sounds like a lot however. Better than laying on the highway I suppose.
Seriously, they have problems with their tank supports and more than a few are known to have sagged a good 4 or 5 inches, and some pop right out on the road. Some factory reps have even made statements in open forum they are not designed to be moved with full tanks which is how they interpret that little yellow sticker that says so many gallons weigh X lbs and needs to be deducted from cargo capacity.......anything works when it comes to skating around a deficiency I guess.
39 gallon tank on our 17 Rockwood, tech had it filled for the PDI. I told him to leave the 3/4 remaining in so I could see how it towed on the 70 mile drive home with water aboard. It barely made it. I was underneath with welder and steel the 2nd day of ownership. I am fairly certain Coachmen uses the same exact supports, which is a standard POS Lippert item.....too light for the load.