His existing tank is an ASME tank and needs to be replaced with the same. The standard horizontal tanks are not made to me mounted under the chassis(it may even be illegal), have to be removed from the vehicle to refill, and require inspection at the same interval as a standard tank. The ASME tank is a much thicker material and made for a permanent installation and does not require any of that.
Manchester is the only manufacturer for ASME tanks that I', aware of. I haven't checked on Worthington.
2011 Keystone Outback 277RL