Yes, they are cheap! Another consideration is how well the limited bay space is used. I initially considered using plastic 55 gal drums until I calculated how much "wasted" space I would be using. I eventually went with rectangualar tanks that allowed me to use my bay space more efficiently.
I am only using one bay for all my water tank and pump, and waste tanks. I also have my propane system, and my 12 VDC and 120 VAC distribution panels in that same bay.
This leave 2 full bays to available to haul other stuff around!
To each his own. I am very cheap, but wasted space must be assigned a cost also.
Tanks are not something you want to have to re-do on the road later.