I find the 454 to be a very good engine. Gas mileage is on the low side but as the old saying goes "if you have to ask about the gas mileage you probably shouldn't buy it"
Our 310 is the second motorhome (1st airstream) with a 454 and we haven't had trouble out of either one. The 1st had about 100,000 and still ran strong. The 310 was rebuilt about 8 years ago at about 85,000 but not due to the engines fault. The PO had a shop work on the A/C and they left the motorhome running and forgot about it. A passer by mentioned the smoke coming out form under the hood. Hence the need for a rebuild.
Parts are readily available anywhere you go which helps a lot. Heck we broke down in downtown Detroit and were able to get parts for it on a Sunday.