I can't say whether you will meet with any interference with your propane tank support frame with the '78 model. My '64 posed no problem with any of the hitch components. What you may encounter is difficulty with the mounting brackets. To my knowledge there are three different sizes relating to the three common dimensions of steel beams used in the "A" frame of hitches on different make/model trailers. For my '64 Overlander, I needed the smallest of the brackets; and on my Minuet the "intermediate" size. Trying to use a size larger than needed for your coach will make the system more difficult to adjust properly.
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)