One possibility if there were two different sizes of Vanagon propane tanks:

Several years ago I think there was a code change regarding hazard protection 
of vehicle mounted tanks - requiring substantial structural change if the tank 
extended beyond certain limits of the body / frame. This may have resulted in 
a change to smaller diameter tanks to avoid more costly modifications.

Propane dealers are paying more attention to "per code" installations, and are 
not supposed to fill tanks on systems which violate codes. While many probably 
still don't pay much attention, if you make a modification that violates codes 
some will refuse to fill it. (Like it or not, it's what the law requires of 
them.)

The code normally exempts older installations from recent changes (not always) 
- but a new tank with different dimensions or changed mounting would probably 
have to comply with current codes.

NFPA #58 has been the national (ANSI) standard for general propane systems - 
but there may be others pertaining to vehicle mounted systems by now. Last I 
knew all states (except Texas) adopted #58 for state regulations. (Texas 
pretty well agrees with #58, but the Railroad Commission writes the state 
regs.)

Sorry to keep bringing up rules and regulations - but I figure it's better to 
know about them than to invest a lot in making changes and then find no one 
will put gas in your system.

Tom