How close can a tank be near an exhaust?

Technically you can fit a tank within 1mm of a exhaust, we say 1mm as we don’t want them to touch as it would eventually wear through the tank, but that would be really bad practice and one we and everyone would frown upon if we say that. W you should do, if the tank is within 100 mm of an exhaust, is to make sure there is some mechanical protection between tank and exhaust.  That can be a cross member, metal heat shield or so forth.

Legally the only comment is in R67 document which states that the outlet / ventilation of the air box on the tank should not allow gas to vent directly on the exhaust - should it ever vent off.

17.6.5.3.         The ventilation opening of the gas-tight housing shall point downwards at the point of exit from the motor vehicle.  However, it shall not discharge into a wheel arch, nor shall it be aimed at a heat source such as the exhaust.

Don't forget it’s not just the engines exhaust you need to consider, it could be the exhaust of the blow heater, the internal gas fire, water heater and so on.